Spindle-To-Pivot Distance

What will happen, if you use a 240mm P2S distance where 250mm is recommended, if you do nothing to compensate for that error, is the null points will move farther apart from each other.  If the tonearm is designed for an algorithm like Stevenson, where the inner null point is already close to the run-out grooves, then there is a danger that the inner null point will now lie IN the run-out grooves or even on the label itself, where of course there is no use for it.  But if the tonearm was designed for one of the other two popular algorithms, the inner null point might still be useful on the playing surface.  That might be no big deal at all, depending upon the LP. But of course also the tracking angle errors before, between, and after the null points are traversed will be altered, probably increased in magnitude. (I haven't done any math on that.)

No big deal. Go to turntablebasics.com and look at their table of mounting dimensions. As long as you have enough adjustability in the tonearm mounting slots you can adjust the overhang and the angle to get correct alignment. For 249.5mm P-S the overhang should be 15.51mm and the angle 20.65deg. For 239.9 mm P-S the OH should be 16.09mm and the angle 21.41deg. Recall eff length = PS + OH. They make a very good protractor.

Why would moving the cartridge forward in a headshell with elongated slots “look ugly”, and who cares?

Do you know what is 10mm ? Try to move a cartridge 10mm forward than usual, it most cases it’s impossible to move that much. The difference between alignment methods is within 2-5mm max, no one have to move a cart 10mm forward in normal situation. And we are talking about different detachable headshells. On a tonearm with fixes shell the slots are much shorter.

I have over 30 different headshells and very few of them (with overhand adjustment like AT Technihard) will allow me to move a cartridge 10 mm forward than normal position which is usually in the middle if the slots. 5 mm is ok, 10 mm forward is too much and in this position headshell is behind the 1/3 of a cartridge body - this is ugly. Normally a cartridge body is under headshell.

Most of the audiophile are excited about ugly things, but in my opinion OP can change the armboard instead and everything will be perfect if he could.

Whether exactly correct alignment could be achieved with the OP‘s situation is definitely a good question. I share your opinion that most headshells would not have that much adjustability room. But that is different from saying not to do it because it would be ugly. Whether  exactly correct alignment is critically important is also another question.

Lewm you brought up the question  that 1st popped  in my head.   Could you move it  10mm  and still have 2 null point.  With the calculator  on vinyl  engine  you should be able  to solve with the variables  he knows.  He could also go to vinyl  asylum  and ask John Elinson (sp)  he has his own spread    sheets.  He could figure this out quickly  and will recommend  to play by the rules.  

As a side note I have been playing around with 2 of my 14 inch arms  with no offset  and 1 null point.  Seems to be a little more dynamic.   If the average distortion is higher  it  is not apparent.   Need to really  sit  down and listen and go back and  forth.   Took up  Kayaking last year  so haven't  been building and listening much.

Enjoy the ride
Tom

PS.  I like the looks  of a cartridge  all the way forward in the head shell.  Although  I  don't remember  the OP  asking  about  this issue.