How much should my 100 pace decline as distance increases?

Is there any kind of formula for what an acceptable drop off in 100 splits should be as sets increase in distance? Let's say your 10 x 100 pace is whatever it is. When one bumps up to say 5 x 200 what % is acceptable to slow on the 100 base? 500 seems to be my sweet spot mentally and physically so that's my goal if that matters. I seem to lose about 8 seconds/100 when I go up to 200's, but then it stays right at my 200 pace all the way up to 500's. Is that normal? Whack? I am re-training my brain to think and train like a swimmer having had the misfortune of coming to swimming as a triathlete. Loving the challenge of getting whipped by swimmers, but as I go away from the old training techniques and focus on shorter/harder I'm hoping to start gleaning some good info from my training so I can know where I'm falling short. Can't get with a Masters group until July-ish so on my own for now. Thanks all.