Disney/Fox is in the driver seat.... Because they own 39% which Comcast wants to own, they can push up the cost for the remaining 61% and if the winner will pay less for the assets than Disney (because the 39% is being purchased for 14.00).
Said another way, For Disney/Fox to pay the same total amount for Sky, that Comcast is currently committed (at 14.75), Disney/Fox can bid 15.23. If Comcast matches that bid (or slightly beats it), Disney could increase their bid to 16.05 and still be paying the same amount Comcast would be at 15.23.
This advantage suggests (Game Theory - My opinion) Comcast will save its best offer for the 3rd round. So, if you are looking at the total cost, and return on investment, 15.23 would be a good opening bid. Of course Disney would want to get it cheaper, so perhaps a bid of 14.76, to force the next round would be warranted (to obtain the highest bid), and force Comcast to show their hand a little in Round 3.
On the other hand if Disney / Fox only want to push up the price of Sky - To recoup some of the $$$ Comcast forced them to pay up for 21st Century Fox, then Disney will want to open with their Best offer, and, Force Comcast to overpay.