Sorry for the lack of detail... I was just trying to prove a point that apparently has backfired and taken a big chunk out of my bum!
The saw is not an issue but a laser light can be a bonus... and for "on the cheap", laser attachments can be bought for most mitre saws... though dual kerfed is a tad more difficult (if they exist, or practical to after fit).
The secret to the jig is that the fence has to be 90° to the front of the base which should be butted up right against the mitre saws fence, still maintaining that 90°... which can easily be lost. Also the jigs fence should be perpendicular to the base. I just used a scrap piece of MDF for the base and some pine (glued and bradded) for the fence. Must have that 90° for the fence else the cut will be "offsided".
When making my flag box, cut the piece to exact length at 90°. Slap on the jig, set the mitre saw to the desired angle (22.5°), butt the end of a piece of scrap hard up against the mitre saw's fence, don't forget to clamp it, align the corner of the scrap using the laser (if available) moving the whole jig sideways as required, don't forget to clamp the jig,
and take test cut(s) moving the entire jig (micro tap is the go) until the edge is a crisp bevel.
Then you cut your required timber and but each end hard up against the fence and cut. Both ends should be crisp if you calibrated with the scrap correctly.
An alternative if you don't have a laser line is to cut in from the edge on a pre-measured piece of scrap, and measure your cut off waste (allow for the kerf of the saw)... alternatively measure the resultant piece and subtract from the original length to get the waste length including the kerf. Add double that length to your desired piece and cut without moving the jig... and don't forget clamping.
If your first cut is too short... Take the second cut from the longer end... LBD
