Gravity is working for you regardless of whether your arch is level or not. Assuming you have the pans connected to each other (siphon or otherwise) they will all level out. For the average user, get it close to level and it's good enough. With restaurant trays - you want them full anyway to avoid burning on the sides so a "slope" doesn't help in any way. As sap evaporates at the front of your arch, gravity will push sap from behind to fill the void.

Only when you start to try to reduce the depth in the pans to the absolute minimum can it helpr. Some folks with a continuous flow arch will elevate the back slightly - that way they are running deeper in the syrup pan than the sap pan. Sap is safe at 1/2" over the flues, but syrup at 1/2" is twitchy at best.

Just get it as close to level as possible.