Sir tooter . . . . . . . .
WOW ! and that's being vewy-vewy heavy on the WOW! aspect
That's a piece of cake . . . after all of the LAYER constructed units that I have either LABORIOUSLY torn down in order to modifiy or built from the "ground" up.
THAT'S being a split bobbin !
You just check out the winding at the right . . . . its secondary . . . and find the wire that is its winding END. Then start peeling off turns and use a 20 ohm 5 watt or more watts . . . . as a load resistor . . . . . while progressively checking and measuring the output voltage, until you finally reach that desired 10V output level.
CONSIDERING that you will need a bit greater than 10V due to different regulator loss, design quirks.
CONJUNCT OBSERVATION . . . . .
That photo and its measured reference of sizing, shows that core mass to be adequate for 5 watts + power handling capability.
That transformer, in its other life, and a ROUGH and unsheltered life it was . . .what with all of those patches of FE O2 deposits, could have actually been a CONTROL transformer of 24VAC out; that would measure as a greater voltage, when being open circuit and unloaded.
Thasssssit . . .
73's de Edd