There's no formula. Again, it's always up to the bull. Your responsibility is to keep your mouthpussy available to him at all times.
All times. He may demand that you suck him off as he's driving across town; you must do so. When he comes home tired from work and needs a pick-me-up, offer to suck his sweet cock. If you're watching television or reading the paper and you suddenly feel his heavy, warm, meaty cock rub (or slap) against your face, then obviously he wants you to suck it. You should see this as an honor.
As for him not lifting a finger, here are a couple examples:
He's watching TV, or reading the paper. You crawl over on your cocksucker knees and rub his crotch, begging to suck his cock. If that's what he wants, he'll allow you to do so. You might also consider waking him up that way -- a lot of bulls really give big points for cocksuckers who wake them up by taking their magnificent, huge cocks into their mouths first thing in the morning. Especially before they wake; you'll find quite often their precious cocks are already hard from a full bladder!
It's quite possible that unless you fail to deliver fresh pussy to your bull, he will make up for that loss by using your cunt mouth for his selfish pleasure more often than he might normally. Again, you should regard this as an honor and privilege. Any faggot can find some other queer to suck; this is a
real man, with a realman cock, and you should be proud to feel his sweet semen squirt against the roof of your mouth, or all over your face (as he prefers).
As for a "percentage of cum that is produced," well, that's a lot of cum in total. Your goal should be to extract
all of it, whether from your wife's sweet stretcarbonsmudged-out pussy, or straight from the bull's throatfucker itself. I don't know which is more satisfying
All I know is, it always tastes scrumptious, and I can't get enough of it. You'll soon feel the same way. A black man may occasionally feel the need to fuck black pussy in addition to your wife's cunt and your mouth-cunt. If so, you should politely ask the bull if he would allow you to clean her out as well. That's the opportunity of a lifetime, my friend ...