Start with your desired temperature, then use our favorite carbonation table to find the pressure to use for the level of carbonation wanted (hint: 2.5 volumes of CO2 is about middle of the road for ales).
Then use the only beer line length calculator worth using. Comes with an education
Or you can pretty much cut to the chase and use 1 foot of conventional 3/16" beer line per the PSI found in the table...