:D I just got a shiny gastly using the breeding technique!
It took me 3 and a half boxes of normal gastlys to get the shiny one, it this considered not much?
Should I try and do it again or will it take more next time I try?
You cannot say for sure... it is still just a chance. You could get shinies every time; there is just a teeny chance of that happening.
Source: My knowledge.