Does she need more feed?

We will show this heifer this summer but are
trying to develop the heifer more as a cow prospect.

She is a legitimate June 18 heifer, not a March papered
as a June.  She currently weighs 750 at 10 months.

Does she need more feed?  She gets 12 pounds a day.

First pic In March, second in mid April.

No. The best and easiest keeping cows aren't pushed at a young age and are allowed to grow. 12 pounds of what?

I have a high fiber grain mix I use.

I agree with Franke. Give her free choice hay, and don't go any higher on the grain mix if you want a good cow down the road. 750 at 10 months is maybe even a little ahead of schedule if she's a June and you want her calving as a 2 yr old.
You'll have to accept that if she's going to make a good cow then she will be underfed/small next to the rest of the "show heifers," but just keep in mind that's not your priority.

I certainly agree that feeding her like a 'show heifer' will absolutely ruin her as a cow.  I have a few high dollar tokens in the pasture as a reminder of what happens when they're fed like that.  Seeing how she's already piling on fat around her tail head, I sure wouldn't increase her intake.  She's shown she's not the type to be able to lay fat evenly so getting her any fatter is only going to detract from her look. 

Thank you for your thoughts.
We will show her a few places this summer but
we don't get real serious about it.
She will be bred for a May calf and turned out after
our State Fair.