Adductor flies: gauging depth

[tyciol]

Does anyone have any ways to gauge the depth of their adductor flies? What I'm worried is as I add weight or get tired that I'll stop going to the same depth. Ideally that's something I want to increase as I get more flexible.

I've been thinking of maybe putting a pair of ottomans (padded foot rests) on either side of my legs, and using them as a gauge by touching my legs to them. Then, as flexibility increases, move them further apart.

Though eventually even those would be too high, in which case all I can figure is a pair of stools, or maybe books. Does anyone use objects like this to gauge their depth for reps?