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I'm just finishing up my first electric build and now beginning to plan my next. I want to make a through neck 6 string but I'm not 100% sure on how to design (make) the neck angle. If anyone knows and has some advice it would really help!

It just depends on the bridge height?

It depends on bridge height and height of the fret board off the body.

Get yourself a big piece of paper and start drawing. Have the hardware in hand or at least the specs for the bridge you will be using. All will become clear quickly.

Most guitars are either copies of the Fender or the Gibson approach. Figure out how you can use your tools to shape that long neck-through wood exactly like the drawing you make.
I'm making a fender style through- neck. I got the top of the guitar and the top of the neck level by sanding with a long aluminum carpenters level with sharp sandpaper glued to it with spray contact cement. You can buy fairly long sandpaper at automotive paint shops. Because Fenders have the bottom of the fretboard about 1/8 inch higher than the flat top of the guitar I am going to make a tapered shim on top of the neck so the strings will hit the fender bridge at the right height. This is easier than it sounds. I'll glue three peices of thin veneer on the neck, one full length, one from fret 5 to 24 and one from fret 12 to 24. Then I'll sand the taper smooth with the carpenters level. I don't run the sandpaper to the end of the level so I can slide that end of the level on a guide block. In this case this is a block of hardwood or steel clamped to the face of the guitar down in the bridge area to help keep the sanding block from rocking and keep the taper controlled. The level is great for establishing flatness. You get a good posture and concentrate on moving that thing smooth and straight.
Because Gibson-style guitars have the whole neck at quite an angle I might hope to saw the long piece awfully close on the first cut, then clean it up with planes or the long sander. If I was nervous about messing up expensive wood I might make a plywood template to guide a router bit with a ball bearing guide to make the edges perfect after sawing a little outside the lines. You can screw the template to the neck piece where it will be inside the body as the body wood will cover the holes.
I hope this helps. You don't need expensive tools but you have to have a definite plan to control the shaping of the wood.