OK

You said that this formula is in L10. That is how I tested

1 - First change the formula to return just the number value, like:

=IFERROR(IF(VLOOKUP(D10,'Sales Stats Comparison'!$D$10:$M$67,10,FALSE)<100%,ROUND(K10-I10,4),IF(C10="",IFERROR((K10-I10),""),"")),"")

, and use percentage number format

This is your formula, just not adding the text part.

2 - you only want the text part if the VLookup() is less than 100%

Add that a conditional formatting condition:

=VLOOKUP(D10,'Sales Stats Comparison'!$D$10:$M$67,10,FALSE)<100%

and for that condition use the number format:

"%TT Not Met"* 0.00%

This will align the text part to the left and the number part to the right.

This is done by the "* " in the format, that will fill the space between left and right. Notice that you can make the cell wider with no problem, the text will adjust.

Remark: maybe your formula can be simplified. I did not get into that.