Generally ... note that I say "generally" ... most project schedules aren't so rigid that the half-hour difference between 8:00 and 7:30 needs to be considered.
And, again *generally*, the difference between a 7.5hr day and an 8hr day isn't important either -- that's why I ask for estimates in work days. It also happens, in my case, that staff work a 7.5hr day and contractor staff work an 8hr day.
Now that I've got that off my chest ... here's the solution to your problem.
You're actually quite close to the solution but I'll post all steps in case others find it useful.
1. Set the displays to show a time:
Tools | Options | View-tab and choose a format that shows the date and the time; click on OK
2. Set up the Project Calendar
2.1 Tools | Change Working Time... click on the Options button; set the Default Start and End times; click on OK to return to the Calendar dialog
2.2 Click on the New button; name the calendar Prj; click on the radio button "Create a new base calendar"; click on OK.
2.3 Select the columns M through F; click on the radio button "Non Default Working Time"; set the From: and To: times as appropriate; click on OK
3. Assign the calendar to the project file
3.1 Project | Project Information...; set the Calendar to "prj".
3.2 (Not strictly speaking required, but do this anyway) Set the Start date to a date with a 7:30 start time.
Note that I didn't click on the "Set as Default" button.