Sorry, I wish it were that simple, but it's not.
Most windsurfing masts are "constant curve" and they bend at a 12% rate.
Then we have MCS (Mast Check System) number designations and IMCS (Indexed Mast Check System) numbers as well.
The top can be pretty soft (like 14-15% and have a stiffer bottom section so you get the 12% overall. Or, the bottom section could be somewhat softer and easier to bend, but the top section could be stiffer. We still get 12% bend percentage.
The MCS/IMCS numbers are simple "deflections" at the 1/4;1/2; and 3/4 distances based on supporting the mast 5 cm from each end and hanging the weight in the middle.
You can get alot more (or less) bend in the top or the bottom, at the same deflection distance with the test weight.
Sailworks and Maui Sails (and other mast/sailmakers as well I'm sure) use a mast test system that actually bends the mast along a fixed track with end loading pressure just like we bend them in our sails.
They've come up with some pretty amazing bends when testing various masts.
So, let's look at what is specified for your 10.0 m2 North Daytona.
At the time the Daytonas were designed, North was on a "minimum mast" program so they designed all of their sails (besides the top of the line Warp Race sails that only worked on top of the line Viper 100% carbon race masts) for only a couple of masts.
A 430 cm IMCS 21-23 and a 460 cm IMCS 24-26. For larger sails like your Daytona 10.0 North sold a CX (50 cm Carbon Extender) piece that you put under the 460 mast to extend it to 510 cm. So my guess (without some serious and detailed testing) would be that your 10.0 m2 North Daytona was really designed for a 510 cm semi "flex top" mast made up from the 460 cm IMCS 24-26 std. mast with the 50 cm CX extender under it.
Think your 490 IMCS 28-30 Yes 55% mast bends exactly the same as a 510 mast with a softer top?
So, since the sail was designed on the 460 mast + 50 cm CX (and maybe even a "drop shape mast" just to make things even more interesting) the tuning marks were added to the sail based on the how the sail was to be tuned on this "stacked up 460cm +50 cm" mast. Your Yes 490 cm IMCS 28-30 mast may be entirely different.
I'd guess stiffer in the very bottom as the CX probably does not bend) and quite a bit stiffer in the top section, so you would be taking alot of draft out of the lower part of the sail in order to get the leech loose in to the tuning marks.
I think my "theory" here is totaly bourne out by your recent rigging experiences.
You downhauled alot less, and left the upper leech tight.
This put alot more draft down lower in the sail because you had been "over downhauling" much more than needed as you tried to loosen the upper leech to the tuning marks.
And your sail rotated alot better and gave you alot more low end power, because the lower panels in your sail were now rigged correctly. The bottom of the sail is where the power is made, the top of the sail adds very little power, but can add alot of "pitchiness" or a top heavy feeling.
Since you are sailing your F-Type in very marginal conditions, I think rigging so you get the best low wind power, down low in the sail, is going to be much better than taking most of the power out of the bottom of the sail to get the correct twist in the top on a mast that does not bend much like the mast the sail was designed on.
Hpe this helps,