I have this deviceon my RTV and B series Kubota tractor
http://www.tiltmeter.com/
They indicate both going up or down a slope and across........For me, 20 degrees gets me puckering and 25 is a "stop what you are doing".........I live and work constantly on 15 to 20 degree slopes and these things are totally a must have tool.
What's even worse is my main pasture is on that 15 to 20 degree angle and the pasture dogs, Anatolian Shepherd, dig bomb craters from a B52 strike while trying to eliminate field mice.........Often times while mowing or working in that pasture across the slope on encounters these craters without knowing they are there.........TALK about PUCKERING.....When the down slope wheel goes down on a 20 degree slope gets one to hangin on tight.........
Anyway, a very worthwhile device if you encounter slopes...........God bless......Dennis