RechargeTime = BaseRecharge/(1+%)
Only use Arcanatime for Cast Times. Thankfully, Mids will do this for you under Options, Configuration, Effects & Maths tab, bottom left, check Use Aracanatime for Animation Times.
Example: Claws Followup Cast Time changes from the normal in game time of .83 seconds to 1.056 Seconds.
To determine how much total recharge you'll need for a given chain, utilize the attack with the highest recharge. In Claws' case, for the chain above, that would be Followup with a base 12 second recharge.
For the chain to work, I calculate how much time can occur between Followups by adding the Arcanatimes of the other attacks. Focus = 1.32 and Slash = 1.584
So I need to get 12 seconds (FU's recharge) down to 2.904 seconds.
Using the equation up top, that gives me 2.904 = 12/(1+x)
Or 2.904 + 2.904x = 12
2.904x = 9.096
x= 3.13 or 313% total recharge in Followup from all sources to get there.
Solve in the same way to get total recharge needed for the other attacks.
I've never done it. With my current build, Followup recharges in 3.11 seconds leaving a horrific gap of .206 seconds which means I would have to deal with the not ready sound fx every freaking chain.
So I silenced that sound fx and now never hear it on any character.
Hope this helps.
Edit: Here's the equation for arcanatime: ArcanaTime = [RoundUp(CastTime / 0.132) + 1] * 0.132
Side note: Something's amiss with the total shown under Misc Buffs for Haste in the latest Mids. It states my build has 286.25% when it only has 186.25% global +recharge. I don't know where the other 100% is coming from. I don't have Destiny Ageless or any of the force feedback +recharges slotted and to verify I disabled all the procs in the build. Edit2: They added in the base 1 at some point. Perhaps they'll revert it.