Glad you're loving it; lovely little guns.
Obviously you're loving its firing manners at lower power and if this output is suiting your needs there's nothing wrong with just enjoying it as it is.
That reading with the TX spring is equating to 10.5ft.lbs. If you were fancying upping the power from its current 6.5, it may be worth trying some different pellets to see what the little rifle is most efficient with (it might well be your current RS) and then reducing the TX spring length a little to get the output and its firing charcteristics just where you want them. Most .177 99s/modern 50s do seem happy at around the 10.5 mark.
Keep enjoying.