In that case, consistency and/or repetition will be your best tools. You won't get a 1-for-1 translation of the sourness after kettle souring vs after fermentation, since (obviously) the flavor profile will change either after your boil or after fermentation. Different flavors could mask the sourness, while other factors could change it.
Without a pH meter, the best way to know how what you're tasting post-souring relates to the post-ferment beer is to repeat the process with the same variables - same bittering level, same yeast, etc. Gaining that experience will give you the tool to say "okay, this tastes 8 on my sour scale post-kettle sour, but i know with this yeast and that hop variety, it typically results in a 6 after fermentation."