You can calculate everything with a pen and pencil. Or you could use a calculator that would speed things up a bunch. I don't know about you, but that's way too slow for me. Computers are too cheap. You don't have to know how to use a computer if you have a good spreadsheet like Excel. You just put in the numbers and up comes the results. My spreadsheet is protected so it's pretty hard to make a mistake. You put in the percentages and it gives you the corners, you put in the corners and it gives you the percentages.
There are actually a total of twenty-seven pages that allow you to do all kinds of karting related calculations. Corner weights, gear ratios etc. etc.
My corner weights spreadsheet is one hundred percent accurate also.