Assuming you set the angle using angle(x1,y1,x2,y2), you could change that to
>Set angle to angle(x1,y1,x2,y2) + n - random(2n)
where 2n is the variance you want. For example, if you want a 10 degree variance, that expression there would read
+ 5 - random(10)
Basically, the plus sets your upper limit, then the minus drops it to somewhere between your upper and lower.
You could also use
>Set angle to angle(x1,y1,x2,y2) + (choose(-1,1)(random(n))
which for some reason pleases me greatly, and means you only have to adjust one number. Again, n would be half of your variance. Mostly I posted the first one first to explain why the thing is happening.
(To other math nerds, sorry, I notated it that way for text simplicity.)