Dosing tanks depends upon the conditioner - each brand has their own measurements.
Over-dosing on conditioner isn't harmful per se, but try to get as close as you can. Over is okay, under isn't.
To be really accurate, I recommend getting a medicine dropper from your local pharmacy or drugstore. It should have measurements on the side, ideally in milliliters (ml) because it's a more accurate way to measure.
Then you do some maths!
1 tsp = 5 ml
5 ml / 10 gallons = .5 ml per gallon
.5 ml x 1.5 (your tank size) = .75 ml total dosage
Though if it were me, I'd just stick in 1 ml instead of futzing around trying to get .75 :)
NOTE: this example is only based on the conditioner you mentioned. You'll have to do the maths again if you choose a different brand.
And for what it's worth, I use Prime :)