You're doing the right things by using the Tiger tuning guide. Setting the spreader rake and mast pre-bend does take patience. You will make adjustments several times...tightening or loosening the side diamond wire turnbuckle, then tightening the main diamond wire turnbuckle on the mast to the desired pre-bend tension. With the mast supported by one sawhorse above the main tang and mast track facing up (spreaders horizontal with no biased loads), grab the mast base and eyeball the mast track from base to masthead. If it is not perfectly straight you'll need to repeat the whole process over again until you get it right. Do this with the mast on sawhorses not stepped on the boat. A rigged mast could have a load biased to one side making a fine adjustment very hard. Once you have the diamond wires at equal lengths (this IS key) with the mast track absolutely straight, making pre-bend adjustments with the mast up is OK. Once you've stepped your mast, take a baseline diamond wire tension measurement with a Loos gauge for future reference.
Check your pre-bend by laying the mast on its side (spreaders vertical..using the sawhorses), then really tighten the main halyard hard (be sure there is no interference with the halyard from the mast top to the mast base) and measure the distance between the main halyard and mast track. Gravity may cause the halyard to sag a bit in the middle but it will still be straight in the axis your measuring. Adjust the main diamond wire turnbuckle until you reach your desired measurement.