I had the same problem on my first outing. On the way home we stopped at Camping World and picked up a few items. One of which was a pair of stick on levels calibrated in inches.
These levels tell you exactly how many inches you need to add under the low side to get level.
When we got home. I leveled the trailer using a 4 foot level on the inside floor. Once it was perfectly level, I stuck on the calibrated levels on the back corner of the trailer near the utility storage where I keep my jacking blocks. My blocks are exactly 1" thick so I can look at the bubble and see how many inches it is out of level left to right before I unhook the tow vehicle. I stack my blocks in front of or behind the wheel on the side that needs raise and run the trailer up onto the blocks. Chock the wheels and I'm half way done. Once I disconnect, I can raise or lower the front end as needed to level front to rear.
My only regret is that I didn't put the front tot rear level near the front so I can see when to stop cranking the jack. With the level at the back, it takes two people or numerous trips between the jack and the back of the trailer to get level front to rear.