There is actually. In the sourcebook Savage Species it has monster levels. It has rules for breaking out all a monster's abilities and features into a pro-rata spread of levels according to it's number of hit-dice. And it even has level progressions statted out for several of the major fiendish and celestial races.
For demons it has vrock and succubus all done up. For devils it has hamatula, imp and kyton. And rules for you to derive your own level progressions for any other fiend.
To get a 3/4 fiend or a partial fiend of any percentage, all you really have to do is slap that percentage of monster levels on top of a base creature.
Say you want a 3/4 succubus, just take a human and add 9 levels of succubus (a succubus has 12 monster levels total). If you wanted to make a base creature of another monster, you just add a quarter of the monster levels of your base creature to 3 quarters of the fiend levels.
Mix and match however you please and you should be able to get whatever percentage combination of monsters that you like.