You'll find that once you get the hang of modelling you'll forget all your sine cos & tan rules, I have, and you'll appreciate being able to get any dimension with a couple of clicks.
It's a long and winding road, learning modelling and also learning how to create useable drawings from your model, do a tutorial or two and things might start to make sense.
In regards to working with 3D objects, IMO "place on workplane" won't help you acheive anything, it has it's uses when working with 2D objects, but for 3D you're better off learning the assembly tools, if you've got them.
If you create a box section by drawing a rectangle then giving it a thickness, when you place it on another workplane the rectangle itself will move to the new workplane, because TC still recognises it as a rectangle with thickness and it's "Entity Coordinate System" (or the entities 'built in' workplane) is based on that rectangle.
If you do a 3D boolean on the same object eg: 3D slice, 3D add, 3D subtract etc. or explode it once, then it becomes an acis solid, or a TC surface (depends if you have pro or deluxe) and its "ECS" moves to the centre of the object. Place it on another workplane and that ECS will align with the workplane, hence 50% above and below.
3D primatives, like box, cone, sphere, taurus etc. have different ECS locations, depending on the type of object.
The 'selection info pallette' tells you what kind of object you are dealing with. Also with "show hide workplane" switched on, you can use 'workplane by entity' to see the entities coordinate system/workplane and you can tell more or less what will happen if you place it on another workplane.
When creating a part that is triangulated in both planes, I find it's necessary to be creative to get a workplane that aligns with one of the facets of the part, then create the part on that workplane or alternatively create the part on any old workplane then use assembly tools and/or the reference point, rotation handles and snaps to locate it correctly. A rail sweep along a 3D polyline is another option, but the results can be tricky to control, and it's only available if you have TC pro.
Wish I could help more, and hope I havn't offered more confusion than clarification. I reckon after few tutorials you'll get the hang and start enjoying it. I remember doing a tutorial on modelling a CRT monitor, in V11 I think (or 9 or 10
), that was pretty good. Still got it here somewhere, if I knew how and that it was legal, I would post it.