QOS: To lower max speed or not

Good morning,

I just noticed on shibby 138 at QOS Basic Settings page it does not have the
"kbit/s (Set to measured bandwidth less 15-30%)"

Should i lower my bandwidth 15%-30% or set it to actual speed?

linksys ea6900
shibby 138 aio
10/.8 fq_codel

thanks