I want to develop some hard hitting 45-70 loads that use the full potential of my Marlin 1895 Guide Gun. I see alot about CUP pressures and not exceeding 40,000 CUP but how can I measure and accurately anticpate chamber PSI for my own ballistic inventions?

There's software, like QuickLoad, that will predict pressure, but only a pressure measuring system like Pressure Trace will tell you for sure. Otherwise, stick to the data in the reloading manuals and the components they list with their loads.

http://www.neconos.com/details3.htm
http://www.shootingsoftware.com/pressure.htm

Bye
Jack

Hodgdon Powder lists full power loads for Marlin 1895 and Winchester/Browning 1886 rifles up to 40,000 CUP. You have to work up these loads, do not start at Max and don't exceed max. They also show pressure for their starting loads, which is nice. Most other manufacturers only list loads up to 28,000 cup for the lever rifles.
Andy
P.S. 40,000 CUP loads in the .45-70 produce substantial recoil, another good reason to work up your loads.

Jack and Andy are correct: unless you have pressure measuring equipment, your only option is to duplicate the tested loads of people who DO have such gear. Such data is in your printed loading manuals.

I'd specifically avoid any "internet" loads. Some well-meaning and even some very experienced people post load data on the web. Unfortunately, so do a lot of doofus types and even malevolent types - and you cannot tell which is which.

Certainly a chronograph can assist the reloader in developing loads from published data. Along with watching for the classic signs of excessive pressure (most of which do not manifest themselves in the Marlin 1895 until one is over its safe operating pressure, but should be watched for anyways) one should not exceed either the charge or velocity of a published load.