'94 Elantra shift points?

Hi, I am kinda new to manual transmissions and the previous two standards that i have driven had tachs to gague when to shift. This evil car does not. It has about 82800 miles on it (not drivin much since april or may and had ~18000 miles put on it since Dec 04). About what speed would be best to shift at for everyday driving? So far i have been shifting:
1-2 at 10mph
2-3 at 25mph
3-4 between 35 and 40mph.
I havent used 5th yet because i have not taken on roads with a speed limit above 50 (i live in vermont)and because the struts need replacing. Am I being to 'nice' to it?
Also, what kind of work would it take to add a tach to it? Thanks in advance for any help.