The line for angle of armor should be drawn from corner to corner over the vehicle, like so...
(I drew the lines from the side of the Tau Broadside because I thought it best to avoid visual complications such as one gun on the front and one on the side.)
You can attempt to do this as you play. Measure for 6" movement toward the Land Raider and measure 6" directly to the side. Estimate the remainder of the distances to calculate the hypotenuse. The line crossing the hypotenuse at right angle is your shortest distance to get in side armor. If the calculations bear out more than 6" so that you can't reach side armor, you have the significant advantage of placing your model elsewhere rather than chasing fool's gold and missing by a 1/2".
Been a long time since I've had to do this sort of math... Geometry wasn't my best.