Unfortunately the description of the problem as landmass ratio is slightly misleading - it does relate to the maximum percentage of water you can have but the percentage varies with the size of the map.

these aren't actual figures but as example if you can get a vanilla or smaller map to work with 50/50 land/water, if you want to double dimensions of map you may have to redesign layout to give, say 75% land 25% water. From personal testing I also think if the water areas are divided in two / three then you can get away with larger amount of water.

I haven't yet seen anyone give exact reasons for problem or exact possible ratios / max sea sizes - I did a lot of trial and error testing drove myself completely round the bend, didn't reach any definite conclusions and gave up and settled on a compromise size / layout!

P.s. I agree - large maps rule