300 seems high to me, realistically. Not like the enemy laner is just going to watch you last hit with Q. Probably fine as a goal, but I think your minimum effective limit should be like 80 in the first ten minutes, and like 120 every ten minutes after. Nasus can totally remain relevant at that level.
Jungle Nasus used to be really popular, and I think the right setup could still work effectively, the difference is you're receiving large amount of stacks somewhat irregularly because of other duties. A flat 100 every ten minutes was my goal, the difference being your first clear should be leveraged into early power, albeit at the cost of a little clear time. Some of my best Nasus games were from the jungle.