DrStrik9 (sounds a bit poisonous to me!),
The issue, regrettably, is that Acrobat is performing as designed. And it has nothing to do with either MacOS 10.10 or anything new or different with Acrobat 11.0.10 (versus earlier versions going all the way back to the initial implementation of the booklet printing feature). And this isn't directly a printer driver issue, either! The same behaviour occurs both on MacOS and Windows.
The problem, simply stated, is that the Booklet print option in Acrobat (and Reader) is attempting to avoid loss of content due to the paper's non-printable margins. The scaling factors in the Size print option are not utilized at all in the Booklet print option. The Booklet print option attempts to fit two logical pages of your PDF file into the printable area of each physical sheet of paper. Assuming that your printer's Letter page is defined as having a printable area of 8"x10.5" due to a 0.25" non-printable area surrounding the page, then the Booklet print option will force each of the 5.5"x8.5" pages of your PDF file into a 5.17'x8" area centered on each half of the 8.5"x11" printed sheet.
There are three workarounds that I know of:
(1) Many high end printers have booklet printing capabilities in the printer or printer driver itself. Check the detailed driver options to see whether in fact your printer / printer driver does support a booklet printing option. If so, try that first. That would likely solve your problem.
(2) Assuming that this is a PostScript printer, edit the .PPD file for the printer you are using to to modify the specification of the Letter printable area. In the PPD file, you will see an entry similar to this:
*ImageableArea Letter/Letter: "18 18 594 774"
or
*ImageableArea Letter/US Letter: "18 18 594 774"
Edit the entry to change only the four numbers as follows:
*ImageableArea Letter/Letter: "0 0 612 792"
That indicates to the PostScript driver that there is no unprintable border on your pages. Then save the .PPD file. I would then suggest rebooting and then trying to print. Note that the side effect of this is that Fit and Shrink oversized pages options will not take into account the non-printable area of your printer.
Note that a similar hack can be done with Ledger (i.e. 11"x17") page descriptions in the PPD to fix a similar issue affecting printing 8.5"x11" pages to 11"x17" sheets as booklets.
(3) There are some very reliable third party plugins for Acrobat that provide full advanced document imposition capabilities, creating what are known as printer flats as a new PDF file from your logical pages. There are n-up, booklet, step-and-repeat, etc. options galore in such packages. Examples include Quite Imposing and PDF Snake. I have personally used Quite Imposing for nearly 15 years for this purpose and can vouch that it works beautifully. I don't have personal experience with PDF Snake, although it is less expensive.
Good luck and let us know if any of this helps you at all!
- Dov