Yes,
I have a 2001 420 with in mast furling, and I put telltales on my main: one set (6 pairs, equally spaced) about 1 ft. aft of the luff (of the completely unfurled sail) and a 2d set ( 4 pairs) about 4 ft aft of (and parallel to)the first set. This (2d set placement) was my guess of the approx. location of what would be the typical "1st reef" point for the main. They work very nicely,and do help very much with trimming the main, although I have not yet had occation to actually reef the sail and check how well the 2d,"reef", set of tells works when the sail is reefed.