I'm doing sub floor repair in my bedroom, and I have been operating the miter saw while it is sitting on the floor. I know I should have the saw up on some kind of a stand or saw horses, unfortunately I don't have either, so the floor is my only option for now and the retractable blade guard is a tad bit inconvenient while the saw is on the floor.. With the guard on I was always more concerned that I would accidentally grab the trigger while holding the guard up so I could line every thing up, my fingers were literally rubbing against the teeth of the blade while holding the guard out of the way, where as now with the guard removed my hands don't have to be anywhere close to the blade.
Here's an image grabbed off the web, like your saw? Notice the long fence:
First off, get up off the floor, it's just not safe. Use an old trashed out vanity, or a couple of saw horses and a piece of plywood screwed down to them. A Stanley Workmate or equivilant is a must have in any shop, less than $50.00:
Secondly, ..... put the guard back on but tape or wire it up with a wedge inserted between the guard and the blade cover. If you don't do this and have the guard totally removed, ALL liability is gone. You are still AT RISK doing this, but somewhat less, just my opinion, but I'm no lawyer.
Having raised up the saw off the floor, it will be easier to see where the cut line is BUT I would make a sacrificial fence out of one long piece of 1 X scrap, bolt it or screw it on, securely on both sides of the blade. Then, cut through it leaving an exact kerf where your cut is. Now, you line up your mark to the kerf, NOT the blade, much safer and more accurate! You can probably leave the guard on without it being wedged up.