Your eye can really play tricks on you when eyeballing grade. I have driven one direction and thought I was driving up hill. Drove the same path from the opposite direction and still thought I was going uphill instead of downhill. We drove along a road after a big rain last year and thought we were driving uphill until we looked at the water in the fields beside us. If you just want to try out the plow and see how it works without spending too much for those short runs, try buying a slope laser ($1-2K) and a magnetic receiver (about $600). These receivers have big lighted arrows that tell you how close you are to grade. (make sure you mount it above the front tip of your plow). You would then have to manually adjust your pitch as you drive. It may not be as accurate as an automatic valve, but it will let you know if your plow is able to hold grade fairly close and if your runs are really short, the tile will still work "okay" even if you are off a bit, though not as well as the best grade would provide. On a 500' run with 4" tile you will likely be just fine unless you have radish roots that find the water in the low spots and plug your tile. If you do not watch your grade close and let it get off and still keep going you could have a real mess. We have Intelleslope and love it, but I understand that for a few 500' runs it is probably cheaper to just hire someone else than buy Intelleslope, but if you already have the plow, it is probably worth it to try a laser and receiver or the old string method.
If you buy or build one of these: http://www.tileholder.com/mini-stringer.php then you can save the cost of a big tile trailer as well.