Err on the side of too long 
The guy lines they have out now are extremely light, yet plenty strong.
For most tarp setups to glass under, you want two longer lines in each corner where your trekking poles are (going to the front) at least two shorter ones low in the rear.
I use these so I can change from a glassing setup to a shelter setup quick and easy
https://zpacks.com/products/lineloc-v-with-loop
The guy lines they have out now are extremely light, yet plenty strong.
For most tarp setups to glass under, you want two longer lines in each corner where your trekking poles are (going to the front) at least two shorter ones low in the rear.
I use these so I can change from a glassing setup to a shelter setup quick and easy
https://zpacks.com/products/lineloc-v-with-loop
