I agree with meanjean, I would start with the 1st gear, Your tractor according to what I read is a 15 hp 2 cylinder diesel, A 6ft finish mower might be just a bit big and running the pto in higher speeds may cause it to bog down, I run a 48" finish mower on my B5100E which is a 2 cylinder 12 hp(10 at PTO), The finish mower has a sticker on the top of the gear case that says to run it at 893 rpm which is fine for grass that is short and dry but if it is wet or higher than 6" I have to run the pto in low speed,I think you will find that you may have to chose a gear to suit different conditions, Your pto sppeds will be like the transmission speeds, On light(short & dry) conditions you may be fine using the higher pto speeds but on heavier(taller and wet) you will need to use the lower pto speeds, You will also need to adjust your ground speed of your tractor which is where your transmission gears and ranges come into play. For me dry/short is Tractor high range 1st gear pto high speed, Wet/tall is tractor low range 3rd gear with pto in low speed.
Hope that doesn't sound too confusing.
Val