The bottom line is there is no substitute for experimentation. No matter what anyone suggests or tells you, the odds are that you will need some amount of experimentation and tweaking to get things they way you want them. IMO, sourdough (I'm making the assumption that is what you mean by "starter") is a labor of love. It's not a matter of following a recipe and having a reasonable expectation of good results. It takes experimentation, learning, and practice.
There are no set ratios for substitution as every starter performs differently from factors that include, but are not limited to, the culture itself, the flour it is fed, the feeding/use schedule, the starter hydration, the ambient temperature, the final dough formula and work-flow, etc. You can minimize some of these variables by following what someone else has done as closely as possible, but even so, you should expect to need to make several rounds of tweaks - maybe more. To the end of giving others a place to start, I've tried to document everything I do - every step - here: http://www.pizzamaking.com/forum/index.php/topic,20479.msg202069.html#msg202069
With respect to a conversion, for my Ischia culture in the 60-75F range, I would say it is approximately: 1% Ischia = 0.015% IDY, 0.02% ADY, or 0.05% CY. If you are using Ischia, this is probably as good a starting place as any. It may be a good starting point for other cultures, or they may be very different.