This is about the best I can think of to walk you through the spreadsheet and the formulas used so you can make up your own spreadsheet but you have to make sure you put everything in the same cell numbers that I've used in the photos below. For cell D6 (which is highlighted in one screen shot) you need to copy the formula that shows at the top. For cell D10 in the other screen shot copy that formula exactly. In column B I've just entered some random figures for rod length, stroke and measured timing so put your own engine's figures in there instead.

What this spreadsheet is all about is to determine how far to raise an exhaust/transfer port by grinding/filing to alter to a desired timing. First you have to use a timing wheel to find the original timings and then strip the engine to measure the conrod length centre to centre of both holes.

In the screen shots you'll see I've shown the timing of an exhaust port as 70 degrees BBDC while it also shows 105 degrees ATDC. For the same engine it should really be 110 degrees because you can measure either from ATDC or BBDC but in this case I've used the ATDC cell to indicate an engine that's had its exhaust timing theoretically changed from 70 degrees BBDC to 75 degrees. Cell D7 shows the distance that the top edge of the exhaust port is from BDC for the standard 70 degree exhaust while cell D11 shows what the distance would be if the exhaust timing was raised to 75 degrees. The difference is the amount that needs to be machined/filed off the top of the exhaust. You can see in the screen shot how I've entered those two figures in the Modifying bit at the bottom. Cell D16 has the formula =D15-D14 to give the answer in mm and cell D17 has the formula =D16/25.4 to convert it to inches.

Just for interest's sake, another way to alter port timings is to machine/file the piston crown at around a 45 degree angle on the edge (matching the port width) by the same amount instead of the liner. This is probably best with engines that use a nickel plated liner instead of chrome (most OS engines) otherwise the nickel can start to peel off if it's not a continuous plating. It's easy to tell if a liner is nickel plated because it'll be plated both inside and out. Chromed liners are only chromed on the bore itself and slightly into the ports.