You need to engage the clutch at the lowest possible speed that the engine will let it engage at.
This will ensure a long life out of your clutch.
Here is a quote right from your owners manual......
"With the engine running, engage the deck clutch switch
(Fig. 3-2) and advance engine throttle to full rpm.
NOTE: Engaging the deck clutch at high engine rpm or
when under heavy load (in tall grass for example) can cause
belts to slip, resulting in premature wear or possible damage."
Later
Pj