American Five Card majors - using Jacoby to respond to a 2NT opening bid
I agree with Aryabhata as far as general principle. Most beginners and even intermediate players greatly overuse Ace-asking bids -- their primary purpose is to keep you out of a slam when you have the tricks but lack controls in side suits; they shouldn't be used to get into a slam when you're not sure you have the values.

A decent agreement in this situation is to use Texas transfers as well as Jacoby. Now you can use the sequences like so:


2NT-3H-3S-4NT: Usually 5332, quantitative invitate to a small slam
2NT-3H-3S-4C: 5-4 or better in spades and clubs (this is mostly to find the best strain, but you can use the follow-on auction to look for slam)
2NT-3H-3S-4S: at least 6 spades, a faint invitation to slam (opener might look for slam with something like Kxx Ax AKQxx AJx -- rich in controls, a good source of tricks)
2NT-4H-4S-pass: at least 6 spades, no interest in slam
2NT-4H-4S-4NT: Whatever your normal Ace-asking bid is (Blackwood, RKC), with 6 spades at least.
2NT-4H-4S-5C: at least 6 spades, first-round control in clubs
etc