Lowering IRS - how many splines

[2fast4u]

I would like to lower the rear of my 72 Super approx. 1 1/2 - 2" and would like to know if anyone has an idea of how much movement per inner and outer spline. I have seen the posts on the "how to" and have the manuals to do it, but I could not find out how much each inner and outer spline is worth in adjustment..... any ideas?

thanks

Robert