Sometimes you need to figure out how much to build something in the future, but you can't figure out exact amounts because IC has so many variables that can't be calculated accurately  (NW, research, empire size, OB cost, etc...) . 

Sometimes you want to build some planest to a certain percentage, can't figur out how many planets you can afford to build to that percentage.

The only thing that it seems you can do is to actually try and guess how much it will probably cost and actually build them.  But you can never build exactly the amount you really want to build.  If you try, it can be extremely annoying: you have to click all the planets you want to build and guesstimate how much it is going to cost, and then inevitably it isn't enough or is too much.  And then you have to go back and click more or fewer planets to build all over again.  This is just a waste of time.  There should be a way to know exactly how much of what infra you can build on the planets you've selected according to the OB percentage you input.

Therefore, I recommend these two changes:

First, after you have checked the planets you want to build, instead of the overbuild to percentage button taking you to another page, it opens a pop-up window.  This way you won't have to go back and check fewer planets all over again when you find out you don't have enough to build or can build more planets at the percentage you wanted to.  You can just bring up your planet page which now still has all the planets checked and simply uncheck or check a few planets and then refresh the OB to percentage POPUP window.  This would make things a lot easier.

Second, there should still be a way to figure out how much your infra build is actually going to cost at a certain point in the future.  So, next to the overbuild to percentage button there is a preview button, which takes you to another POPUP window which is exactly the same as the overbuild to percentage POPUP window but instead of actually building the planets it just tells you how much it will cost to build the planets you have checked to the OB percentage you input.  This way you can know exactly how much builds are going to cost and can begin to figure out how much you should ACTUALLY save for infra jumps.