How do I figure the cost basis and capital gains on the sales? -->
When a company spins off a division and you recieve shares in the new company relative to the number of shares originally purchased, you must split the original basis between the old and new shares. Most company websites will have information on any stock spinoffs they conducted, including what percentage of your basis should be assigned to the old/new shares. Alternately, there are a couple of websites that have calclulators for this sort of thing, with data on many companies. Try this one: Spinoff Calculator
Don't forget to include comissions (if any) in your cost basis (which are also split if the were pre-spinoff).
I was hoping I don't just claim 15% of the total of the two sales and pay that? ->>
From a practical point of view - Since you held all of the shares for more than a year and have sold all of them (in the same year?), you should pay no more than the maximum long-term capital gains (LTCG) tax rate (5% or 15% depending on your income) on the difference between total sales price and cost basis for both batches. Schedule D requires you to record the individual sales so you still have to split the basis as noted above - just make sure you don't pay more tax than:
total sales price (both batches) - total basis (amount you paid in to acquire shares, including commissions) * LTCG rate