I think the instructions mean to say that the glue is applied 2.25 in. from the end of the booster motor tube. Then the thrust ring and spacer tube are inserted so that .25 in. of the spacer tube is extending out the back end of the booster tube. So the thrust ring's final position is 2.5 in. from the end of the tube. Make sense?
Or to put it another way apply the glue 1/4 inch short of where the thrust ring will finally end up. When you insert the ring and motor spacer tube the ring will contact the glue just before reaching it's final position. Refer to the lower right portion of the "Assemble booster engine mount" panel for a good schematic with all the dimensions.
Have fun!

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I like the unusual, the off beat, the avant-garde. No 3/4 FNC for me!