I wish there was a simple answer to your question, a simple formula, but the keel design and weight is just a part of the overall sailboat design. The hull size and shape, mast designs and amount of sails to be carried are just parts of the design. I don't claim to be an expert on sailboat design, not even close. Too much to try to cover in a blog.
There are many great books on this subject and design and research continue today. It is fascinating and I started on the model boats aspect in about 2003 and continue to learn each day.
Even this boat, based on the Star 45 design, does not give a specific bulb weight, just a suggested range. Once the boat is completely assembled with sails in place there are balance tests that I do, I'll take some photos at that point. The bulb weight may have to be adjusted at that point. I already had this bulb on hand so it's a start. At 6 lbs I suspect it may be a little light, we will see.
Thanks for your interest.