You can give her a flat rate. As I said it is the same thing in the end. A 8 in cake that serves 24 people is either $48 or $2 a serving. (Using $2 as an example.) So you can tell her $2 per serving or you can tell her $48. They both mean the same thing. I always give out both, that away people are aware of how many servings they are getting. Especially if they don't know for sure what they want, and I list a couple of different options.
A personal pet peeve is people that have 'flat rates' that don't make sense. The ones that have $25 for an 6in, $30 for a 8in, and then $40 for a 10in. When you price per serving them, the numbers just don't add up.
6in - $25 (serves 12) is $2.08 a serving
8in - $30 (serves 24) is $1.25 a serving
10im - $40 (serves 3
is $1.05 a serving
When people that that, to me there is no rhythm or reason they have the prices they do, other than they pick them out of the air.
Another thing to note is that you don't have to do the cost per cake for every cake you do. Find the average and work from that. If you do a lot of 6 and 10in cakes, then figure the cost and time involved in that one, including your supports and such. Then divide it by the number of servings. At that point you can adjust the price per serving up to a rounded number and go from there. Then no matter what the size you have a number to work off up.
For me the price per serving makes math easy. As long as I know the price per serving and have the serving size chart I can price any cake. And it allows you to be consistent in pricing.
You can always do the math and have your 'flat' rate written down beside each cake size.