help with dynamic block

Im creating my first dynamic block. Ive got the basic geometry of the block all i need to figure out is how to stretch two lines equally in opposite directions. ive attached the block that ive got so far. pretty much it represents a fire smoke damper, i want the circle in the center to stay the same size and the lines on either side to be able to stretch equally so it can be used for different sized ducts. any help would be great

FSD.dwg

try this:

FS1

you could set min/max lengths or use other parameters for it...

that works perfectly, thank you so much... As soon as i get some down time i need to figure out how to make dynamic blocks

Just play around with it in the block editor - you'll figure it out.

Like I said, check the options for MIN/MAX and distance type in the properties for the distance parameter while in the block editor...