user parameters fusion 360 not allowing me to set formula by referencing formula values set letter form

[alexwrathbrozen]

( 2 * U_ ) won't let me add this  U_ = 0.254mm but will let me add this ( 2 * 0.254mm) I have set U_ user parameters so won't tie other users' parameters in as values if they are not number 

[jhackney1972]

You last statement needs further clarification of what you are trying to do or mean.  The other statements work find for me.  Post a screen capture of you parameter table to show what you are trying to do.