Firm starter to liquid starter?

it's not going to be a big deal, the point is to use up your excess starter mostly. To switch over your firm starter - I don't remember what hydration the MG starter is but let's say 50% - you know that to make it up as liquid starter at 100%, you need to add water. In this case, you need 50% of the flour weight more water. 

So if a recipe wants 2 cups of liquid starter (why are King Arthur still not doing weight?!?) you know that's going to be about 280 per cup of 100% starter (according to Mike Avery). So that 2 cups will be 560 grams, half of that flour. Going with your 50% starter (assuming) if you have 280g of flour, you only have 140g of water - total of 420 grams. You therefore need to add another 140g to the recipe.

Personally, I'd have a very hard time collecting 2 cups of excess starter since my excess is what I use to make bread; I only get 40g of it per feed. I'd likely just "build" that 580g or else it would take several months to collect that much.

If you decide to go the make-it-on-demand route, it should be easy to mix up 560g of starter from scratch. Weigh a couple tablespoons of your stiff starter - say 30g. deduct that from the 560 they want: 530. Divide by 2 : 265 g of water and 265g of flour. Mix that with your stiff starter in a big bowl/jar (it's 2 cups but will double or more) and let it sit out overnight. By morning, it should have populated the 2 cups and be ready to make good pancakes.

Paul,
http://MellowBakers.com
A Hamelman BREAD baking group

Hydration has a formula...water weight divided by flour weight. 

If you take a tablespoon of starter that's thick and add 100g water and 100g flour it is very close to 100% hydration.  

You want 70%?  Take a blob of starter and blend with 70g water and 100g flour.  

For 166%   ... 166g water with 100g flour.

Mini Oven simplified it in another posting, as noted above.

Paul, KAF does offer their recipes in both weight and volume.  You just have to select the one you want by checking the proper button.