pushrod length ball to cup

Hi all!

I have 471 BB in my Duster and wanted to go from stock rockers to 1.6 roller adjuster rockers. Should get more power with my heads according to flow chart.
My pushrod length now is 8.575 meaning stock with two ball tips. I have hydraulic cam.
Is there a formula to determine what length ball/cup pushrod should I get based on ball/ball pushrod?

Normally I would not bother you guys and just checked after getting rockers here, but I am in Europe and usually wait 2-3 weeks for shipping of parts... Knowing what to get would give me almost one more month of fun before winter comes... :burnout:

Thanks for your help.