1) Go to your local Home Depot and buy one of these >>>
Husky 3/8 in. Click Torque Wrench-H3DTW at The Home Depot
2) If you don't already have one, also buy a 9/16" or 14mm 6-point socket for a 3/8" drive ratchet wrench.
3) Adjust the torque wrench to 40 ft-lb, and tighten the blade bolt until you hear and feel a click in the torque wrench.
You can also use the torque wrench to loosen the bolt if you set it to a higher torque value and reverse its setting to work in a counter-clockwise direction.
Make sure you read and understand the torque wrench instructions or you will ruin it.
Do not use an adjustable (crescent) wrench to loosen or tighten the bolt as shown in the EGO manual, or you will likely ruin it.
Always use 6-point sockets, rather than 12-point sockets, as the latter tend to ruin hex bolt heads.
Torque is a force applied through a lever arm perpendicular to the force. So, if the the distance from the center of the grip on the torque wrench to the socket is 1 foot (1'), then 40 ft-lb will occur when you apply a 40 pound (40 lb) force to the grip of the wrench.