help with IR on arch

[davismetal]

I am building a sign and getting ready to order pre bent 3x2 tube steel.

they need to know the IR that i want.

I need 3 20 foot pieces that i will be putting 18 inch letters between in 2 rows.

i dont want a rainbow shape more of a minor arc. do i need a 18" ir?

[txcncman]

I just answered this on another thread.