I've never tested this but I think it would be this way:

First go to options and select 3000 lines YENC as the method for restricting block size to multiples of the unit. Make sure the program you are posting with is using 3000 lines and not 5000.

Here we have added 69 rar files at 100MB each. Slide the block count slider all the way to the right and then select sizing scheme all the same size. At this point you could use the redundancy slider to select percentage and also set number of recovery blocks and recovery files. This is not really precise though. If you want exactly the number of blocks needed for 7 par2's at 100 megabyte filesize then I think you can take 274( number of blocks per rar file) and multiply it by however many recovery files you want. So 274 blocks X 7 Files = 1,918 blocks. Plug this 1,918 number into the recovery blocks field and it will give you 7 recovery files at 100 megabytes.

Once again, this is untested but the best way to find out if it's wrong is post it and somebody will let you know lol.