Hello John, good to see that you are still around.
The design program follows rigid numerical rules that make sense most of the time but perhaps not all the time.
In addition to the 300 gr. minimum for the XL block, there is yet another rule about minimum band size for each caliber. The program displays only one warning at a time even if multiple rules have been violated, so you may address one violation only to discover that another rule violation is waiting for you.
The lightest bullet it would let me make with your 90% meplat and 0.250" long nose was 270 gr.. It's bands were 0.0787" long. How long would the bands be on a 230 gr? Well, it won't tell us but I bet they would not be long enough to make the program happy, which requires a minimum of about 0.075" long bands for 500 caliber. Plus, we are still stuck with a bullet that is too light for the XL 2-cavity and too big in diameter for the standard 2-cavity.
50 caliber definitely does not fit comfortably in the XL 3-cavity, and 230 gr. will cast very poorly in the 2-cavity XL. I have occasionally bent the rules to put light 50 caliber bullets in the standard 2-cavity and that is what I would suggest in your case.
That still leaves us with the question about band layout since the program will not even allow us to create a 230 gr. bullet with a decent lube groove.
By trial and error, I was able to create a 230 gr. with 90% meplat providing I shrunk the nose length to 0.190". That shifted more weight to the bottom half of the bullet so that the bands would meet the 0.075" minimum. A 0.7 GtoB provides an adequate lube groove.
If you want to sketch some alternative band layout, I might be able to cut it, or not, just depending. I'd be willing to go as small as 0.060" bands on your light bullet. I'm guessing that even 0.060" long bands would still exceed 230 grains with your original 0.250" long nose so it seems like some nose changes are necessary to hit 230 gr. -- either a smaller meplat or a shorter ogive to reduce weight on the nose.
As for the block issue, I would be willing to put a light 50 caliber in the 2-cavity standard block. Once we agree on a design you can either snail mail the order or else I can send you a paypal invoice.