Parti-gyle questions

I am planning on making two beers out of one mash but want them both to be a full 5.5G. The first will be a double IPA on the weaker side(about 1.080-5) and the second will be a blond/pale ale(depending on grain bill). Does anyone have a guess what percent of the total sugars will be able to be sparged off through multiple batch sparges?Should I assume that I'll leave about 5% in the mash....or more? I was also wondering if the easiest way to attain proposed gravities is mathematically or just sparge it all out in separate vessels and then combine to get right gravities? I guess I don't want to spend time figuring the math and then have the end result be off several points. Has figuring it worked well for anyone?

With first runnings and two sparges (all equal volumes), my numbers work out 50%, 33%, 16%. If I was doing this, I'd split the sparge water into three batches. First runnings and first sparge in the IPA.

I recently did a similar brew but with three beers. Your first runnings are going to be high. Here's the thread, check out the results in post #28.

https://www.homebrewtalk.com/showthread.php?t=41419&page=3

I just crunched the numbers on my above post and here's the percentages for each of the 3 worts:

1st Runnings - 62.5%
2nd Runnings - 25%
3rd Runnungs - 12.5%

Here's a good reference:
http://www.strangebrew.ca/swig/index.html