How do I find out the fuel consumption of the PT6A for a lightly loaded aircraft?
You need to obtain the specific fuel consumption (SFC) for the particular dash number of PT6 (scroll down to the bottom for the chart with the data). For example, the -67A turboshaft is down around .47 lb/hp/hr, close to piston engine territory, but most are between .5 and .6 lb/hp/hr.
So pick the dash number of engine applicable to the airplane you want to compare, note its SFC, and now all you have to do is calculate the HP required of a given aircraft at a given weight to cruise at a given speed, and that you should be able to interpolate from the airplane AFM/Operating Manual's cruise performance and power required chart data, working out the HP required to cruise at Speed X, and Weight Y.
Once you work out the required HP, and you know that engine's SFC, you're there.