Canvas expansion incorrect, percentages don't match instructions

OKAY- THIS MATH MAKES SENSE.  Thank you! I can use this for my action.  I jsut got through a call with Adobe support and they were basically like, no, but thank you for helping.  I appreciate your sticking with me.  

Yes, of course. There it all fell neatly into place.

4785 x 1.18 = 5646

Your new canvas width is 5646 pixels wide.

Put that number into Canvas Size. It will add equal amounts on either side.

Maybe a better question is:

How would i use math to get 9% margins on either side of my canvas if i start at 4785.  What would I type in for the canvas width? 9+9= 18, so i would think 118% would give me the result i need.  Since you keep telling me the math is correct, help me see how typing 118% doesn't give me my desired result?  How would YOU get 9% on each of the toe and heel?  I feel insane, but we just keep talking around each other.  I don't want 7.6%, i want 9%.  Thanks

Your maths is incorrect.

4785 x 1.18 = 5646.3 which rounds to 5646 as Photoshop can't add half pixels.

That is 5646 - 4785 = 861 additional pixels

861/2 = 430.5 at left (and 430.5 at right)

The left guide will now be at 430.5 which is 100 x 430.5/5643 = 7.6%

The right guide will be at 100 x (5646-430.5)/5646 = 92.4%

Photoshop is working correctly

Dave

Hello-  Can you look at my screenshots?  What youre suggesting is exactly what i've done, but the result is not what you'd expect.  I calculated that then added it manually to the left side only, repeated for the right.  But again, my shoe is filling too much of the frame after that expansion.  It's like the shoe layer sizes up?  I ended up experimenting and wrote an action that crops tight to my shoe (4785) then expands the canvas to 6635, a difference of 1850 pixels.  1850/2= 925 pixels.  Doing this puts my guides (at the heel and toe) at 9%, but 4785*.18= 861 pixels. I don't understand why i have to arbitrarily come up with a pixel expansion to get my guides to sit at 9%.  I should be able to use math.

Well, it's easy enough to find out. Note the pixel width before and after, and then open the calculator and multiply original width by 1.18.

Hi! I have my ruler's set to percentages, when the canvas expansion is done, the guides are at 7.6% instead of 9.  If i've cropped my shoe tight, that means the canvas becomes sized 100%.  That is 100% of the canvas size.  So then when i say to expand it by 118%, i would expect to see the shoe, plus 9% each on the heel and the toe.  Is this incorrect thinking?  I swear i have done this before, also it is logical.  Def let me know if i'm incorrect here, but then also how you would do this- have the shoe cropped (that's your 100%) and then what canvas expansion you would do to get 9% each on heel toe.  Thanks

How do you decide it's less than 9%?

If the number of pixels is correct, then it's correct.