I just finished doing this
Keep the iterative cuts to less than 1/2" each time and it will be hard to miss.
First cut can be eyeballed. The raw baffles have steps at the joints. Place the top cowl roughly in place (no jigging) and look through the inlets to see the gaps. Goal is to remove the gross steps.
Remove top cowl and roughly mark memorized spots for a 1/2 inch cut. Execute cuts.
Maybe do this eyeball one more time (since it is quick) if you still have locallized big gaps.
Like Mark said above, make a marking stick. I used a medium sharpie (not the fine one) with a 1" grommet taped to the tip so that I had a 1/2" offset no matter how I held the sharpie. Tape that to a stick to reach into the cowl. Mark the areas you can. Remove cowl and join marks by eye or straightedge. Cut.
It took about 3 iterations of this to get to a perfect fitting 1/2" cowl gap. On the last marking iteration, the cowl should fit fully in place before the final marking. Again, if you keep the initial cuts below 1/2" each time, and then start following the sharpie marks this should happen automatically.
If you have an IO engine using the snorkel, DO NOT HACK THE LEFT RAMP BEFORE READING THE SNORKEL FILTER INSTRUCTIONS.
I was lucky. I did cut the left ramp just enough to get the lower cowl on. Just before whacking it again per the baffle instructions, I whipped out the filter and plopped it in place. Yikes! My cut marks would leave a ramp smaller than the filter! The cowl inlet must be cut instead. Be careful.
CLARIFICATION: The marking stick method worked best through the spinner hole in the cowl. I did not have the starter ring gear or the front center baffle sections in place during the procedure.