I used the thrust to weight ratio of 2.12 to determine the thrust my spacecraft needs for a lunar liftoff, Apollo Lunar Ascent Stage Thrust to Weight Ratio. Is this a proper way of determining and sizing my spacecraft based on the Thrust needed using the thrust to weight ratio of the Lunar Ascent Module?
You don't need exactly 2.12:1 thrust-to-weight; if you have less, you'll need a little more ∆v to reach orbit, because you'll lose more altitude to gravity on your slower ascent, and if you have more, you'll need a little less ∆v. Obviously you need something more than 1:1 or you won't move until you've burned off enough fuel to drop your weight below your thrust.
The exact performance you need is dependent on a large number of engineering factors, but around 2:1 initial thrust to weight is good rule of thumb for lunar liftoff.
(Taking off from Earth you can get away with something like 1.2-1.4:1 initial T/W instead, which is comparable in initial acceleration.)
For a 45 ton spacecraft, to produce 3.4m/s^2 of acceleration (about 2.1:1 TWR), Wolfram Alpha tells me you need 153kN of thrust.
You can check this against the Apollo Lunar Module ascent stage, which was about 4.7 tons (10.4% of the weight of your spacecraft) and had an engine producing 16kN (10.5% of the thrust of my answer).