Blender's version of Sketchup's line tool? (I previously opened a multi question thread, which is against rules, so here's just the main question I had)

As a relatively long term Sketchup user I can't work without the line tool and the Knife tool in Blender doesn't show distance and it doesn't snap by length like the Sk-u line tool, so it's really hard to work with it.

Basically here's what I want to do, to draw a small line 20cm from ,,point Q" (random name I thought of) and then to draw a line towards the other side of the box to the point that is also 20 cm from it's version of "point Q", hopefully without using subdivision and loop tool, which both create a lot of unnecessary things and are both unprecise for what I need.



I was advised previosly to use the snap utilities addon, but I only want to use either Blender's own stuff or at least a freely available addon (snap utilities is 17 usd), I am a 21 year old from east europe, so I neither have (or need) a credit card, nor a paypal and I'm not in the mood to get those 2 just for a small snapping addon, I'd probably end up paying a lot more for the credit card itself..., so if there is anything free, it would be much appreciated.
Blender isn't really the right tool for doing precision 3D design, but it can be done.

You will have to do some arithmetic, but one approach would be


start with a view perpendicular to the face you want the edge on.  In your example, it looks like that would be top orthographic, so hit NUMPAD-7
select the knife tool (Shortcut KKEY)
place the cursor on the either of the edges you want to connect and click LMB
type the C key
place the cursor on the other edge and click LMB
type ENTER


This will create the edge you want, but not place it where you want it.


type g
do not move your mouse
Go to transform panel of the properties panel and enter a value.  (if the properties panel is not visible, type N to unhide it) the transform panel looks like




To figure out the value, here is an example, based on your image.
* You appear to be adding the edge to a top face.  This means that you want to set the median y value entry.
* take the y value of point q and subtract 20cm from it.
* enter the result.

You end up with a precisely place edge.