Tongue weight is really only valid when measured at tow height. Tow tongue down or up and the fulcrum effect takes place. You will either add tongue weight from the axle forward, if towing "down" or lighten the tongue weight if towing "up." This measurement can be critical if a front bathroom model or water tanks up front.
Don't take just any ole tongue weight on your trailer and consider it valid for your tow application. Of the two, if you can't tow level, tongue down is preferred. The main concern is you aren't introducing "sway" by having the tongue too
light, hence the 10%-15% rule.
You can cut a solid piece of wood to length (for your tow height which you've already measured), place it under the coupler and lower the jack until the wood is sitting on the scale. Continue to crank the jack up until it just clears the floor.
Most scales only going up to 250 lbs, so using a scale in this manner won't work to measure tongue weight higher than that.