Nick, welcome back. There are many variables that need to be accounted for before anyone can offer accurate advise. Knowing if the motor is set up to be rear mounted or front mounted would be the first thing to know. I'm going to assume that the cowl has been previously mounted. This should actually be a help. My suggestion would be to first determine if the firewall has any offset for right thrust or not. Some models will have it built into the firewall. If yours does not I highly suggest you add it. This will make takeoffs much easier. This can be done by just shimming one side of the motor mount with some nylon washers. Once you have determined how far the motor needs to sit off the firewall, add the appropriate length stand offs. To figure out standoff length, mount cowl and measure the distance from firewall to where you want the prop drive washer. Suppose that dimension is 7". Now measure the distance from the back edge of the mount to the face of the prop drive washer. Suppose that dimension is 4", then your standoffs need to be 3". You can fine tune with washers. To get the proper mounting hole location, stand the fuselage vertical, place the motor on the firewall and mount the cowl. Position the motor so the shaft is centered in the nose ring and then mark the firewall where the standoffs or mount contact the firewall. Don't forget the right thrust, that will require you to move the motor off center slightly.