I believe that the number of days off on your May "pay only" line also has to be protected on your May "custom line". Not the actual days off. But, the number of days. It doesn't matter if you have actually have any training days in the month or not. The key is that you're being built a "custom line" for May and you have bid a "pay only" line for May.

SEC 25. I. 3. b. i.

A pilot returning from training shall be credited an R-day value for each day he receives or shows for scheduled training other than IOE, and the higher of SCH or ACH for IOE trips. The number of days off on such pilot’s pay only line shall be preserved when constructing his custom line. A day off during his training footprint shall be considered a day off when constructing his custom line.

I'd go for max days off. Less reserve, unless you live on the planet, there.