Then just use http://bitcoindifficulty.com/ w/ history of percentage difficulty increase and plug into calc.
That's what I already do, but who knows what the difficulty is really going to do once all these ASICs get on the network (plus all the backup hash that ASICminer et al. are keeping from us at the moment.
I also use https://bitclockers.com/calc which gives a better ROI estimate. Anyone got any other calcs they use?
I use this calc http://www.alloscomp.com/bitcoin/calculator & these "estimates" of difficulty @ corresponding network hashrates,have fun!!!
19707600=150TH
26276800=200TH
39415200=300TH
52553600=400TH
65692000=500TH
78830400=600TH
98538000=750TH
Just copy & paste into the diff box