Kitchen Island Help

[Halfathumb]

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This is my new island. I'm using Blum Tandem Plus Full Extinction slides.  I'm afraid I made a wrong decision in the construction process.
I screwed the glides to the face frame with anticipation of making the drawer sides fit to the drawer face with BDT. But when I really got into making the drawer faces with dados around the inside of the face so it will fit over the outside of the drawer opening. Realized the drawer probably won't work properly.
I can rather easily fill in the 3/8" gap behind the face frame and move the glides back a little.

My question is to those who have experience with these glides. Do I make the drawer sides exactly 21 1/2" as the entire glide is or just the 19 1/2" as the sliding part goes?

I know this is a silly question but I don't to waste more time & lumber.

Thanks, Jim

[Splinter Puller]

The critical dimensions are the spacing from the insides of the drawer sides and the drawer side thickness.  I use sides which are 1/2 or 5/8" and there is usually a 1/4" or so clearance between the drawer sides and the face frame.  

Did you download an instruction and construction guide from Blum?      The first time I used them making a test drawer really helped.

[Halfathumb]

The critical dimensions are the spacing from the insides of the drawer sides and the drawer side thickness.  I use sides which are 1/2 or 5/8" and there is usually a 1/4" or so clearance between the drawer sides and the face frame.  

Did you download an instruction and construction guide from Blum?      The first time I used them making a test drawer really helped.

I'm probably being way to cautious with this. I've lots of drawers, most with hand cut DT. I will take your advise and make a test drawer first. I've never used Blum guides before. I guess I'm kind of a fraidy cat.