This is my (incomplete) version of a d20/brp conversion guideline. The focus is on converting statblocks (especially npcs) as easy as possible.
Definitions
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BRP Base Percentage of a Skill = BP
D20 Skill Bonus = SB
Ability Scores
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Strength = Strength
Dexterity = Dexterity
Constitution = Constitution
Size = the effects of Siz are part of Con in d20, I would either simply use Con, or use a formula that allows only a minor variation (like Siz = Con -3+1D6)
Intelligence = Intelligence
Power = Wisdom
Appearance = Charisma
Education = Intelligence + Wisdom / 2
Skills
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In d20 you usually have a statblock that includes only the total-skill-bonus (at least for NPC). It would be a lot of work to look for all the boni that may be included (Attributes, Feats, class, synergy). I would take the SB as it is (just exclude potential magical boni) and convert it with the following formula:
BRP Skill = BP + 4*SB
for example: Appraise +7 = Appraise 43% (15+4*7), Bluff +7 = Fast Talk 33% (05+4*7), Diplomacy +16 = Persuade 79% (15+4*16) (the table with related brp and d20 skills isn't ready yet).
That way you can ignore everything (except magic) that gives you skill boni because it is already included. Additionally you have converted the skills without ignoring the BRP-system-specific difficulty of a skill. And finally: it is so easy, that you are able to write a small program/or excel table, that does the complete skill conversion automatically without going into d20 rule details (and that would take some programming work). I know that the Attribute-boni to skills are still included and I would do it this way because a) it's easier, b) you often distribute skill points in d20 to get a certain bonus including attributes (and you would get weaker in the area in brp if you don't take that into account), and c) if you take only 4*SB (instead of 5*SB without boni) it doesn't make a lot of difference for the characters with better attributes. I think the resulting percentages (see example) are in an acceptable range, not too high for advanced characters and not too low for beginning characters.
Some d20-skills may have the same converted BRP-skill (spot and search (d20) would both convert to Spot (BRp) for example). In that case I would use the skill with the higher SB as a base and add 1*SB from the lower skill. The formula would be:
BRP Skill = BP + 4*SBhigh + 1*SBlow
for example: Spot +10, Search +8 = Spot 73% (25+40+8)
On the other hand, some d20-skills may convert into more than one brp- skill. If you think that would make them too powerful, you could use the following formula to make each brp skill a little weaker:
BRP Skill = BP + 3*SB
Languages: This is a little more difficult, in d20 it is assumed that you speak a language well, if you have the skill - so it is probably necessary to calculate Language (Other) in another way - maybe Edu*3 or something like that.
Weapons
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Same as skills, but you use the attack bonus of each weapon as a base for the calculation. The formula is:
Weapon Skill = BP + 4*(attack bonus with weapon)
for example: ranged+5 and Weapon Group Proficiency (blaster pistols) would convert to blaster pistol 40% (20+4*5)
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= Example d20 (star wars)-statblock =
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Male near-Human, Noble 2/Jedi Councilor 5; Init +1; Defense 18 (+7 class, +1 Dex); Spd 10m; VP/WP 49/14; Atk +5 melee (2d8+1/19-20, lightsaber) or +5 ranged; SQ Call in a favor, inspire confidence +l; SV Fort +6, Ref +6, Will +9; SZ M; FP 3; DSP 10; Rep 8; Str 13, Dex 13, Con 14, Int 14, Wis 14, Cha 15. Challenge Code: D.
Equipment: Lightsaber, personal transport
Skills: Appraise +7, Bluff +7, Computer Use +3, Diplomacy +16, Gather Information +9, Intimidate +7, Knowledge (Firrerre) +7, Knowledge (Jedi lore) +4, Knowledge (Sith lore) +6, Profession (merchant) +7, Read/Write Basic, Read/Write Firrerreo, Search +4, Sense Motive +9, Speak Basic, Speak Firrerreo.
Force Skills: Affect Mind +10, Empathy +7, Force Grip +6, Force Stealth +7, Friendship +7, Move Object +7, See Force +8.
Feats: Force-Sensitive, Sharp-Eyed, Trustworthy, Weapon Group Proficiency (blaster pistols, simple weapons).
Force Feats: Alter, Control, Deflect Blasters, Sense.
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= BRP =
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Male near-Human, Noble/Jedi Councilor
Str 13, Con 14, Siz 14, Int 14, Pow 14, Dex 13, App 15, Edu 14
HP 14, Move 10
Weapon: lightsaber 30%/30%, blaster pistol 40%, Grapple 45%, Brawl 45%
Skills: Appraise 43%, Bargain 32%, Bluff 33%, Computer Use 12%, Etiquette 41%, Knowledge (Firrerre) 29%, Knowledge (Jedi lore) 17%, Knowledge (Sith lore) 25%, Language Basic 70%, Language Firrerreo 42%, Persuade 79%, Sense 46%, Spot 41% (Intimidate +7 - maybe a +7% bonus on persuasion)
Force Skills: (can be converted as skills - BP 0% is assumed here) Affect Mind 40%, Empathy 28%, Force Grip 24%, Force Stealth 28%, Friendship 28%, Move Object 28%, See Force 32%
Equipment: Lightsaber, personal transport
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Not Converted
everything star wars specific including Weapons; Feats: Force-Sensitive; Force Feats: Alter, Control, Deflect Blasters, Sense.
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