Here's some free advice.

First, if you were to design such a tartan, I wouldn't name it "MacFalconer" because that is a name that does not exist. If the intent is to design a tartan for the Falconer family, then it should be called Falconer. There is no such family as MacFalconer.

Secondly, there already is a Falconer tartan. It's the same as the Keith, Austin, and Marshall tartan. The fact that the tartan shares more than one name does not make it any less legitimate. (There are many examples of shared named tartans: Hunter/Russel/Galbraith/Mitchell, Black Watch/Campbell/Htg. Grant/Htg. Munro, Campbell of Cawdor/Argyll district, Grant/Drummond, Drummond of Strathallen/Ogilvie of Airlie, Kidd/MacPherson, etc.)

In this case, the Keith/Marshall tartan was adopted by the Falconers when the Lord Falconer took over the lands of the Keiths, Earls Marischal, in the early 1800s. So the connection is quite legitimate.

There is also a newer tartan called Falconer of Labhdal that was designed in 2005, and based upon the Falconer/Keith tartan. It has two blue lines on the green field, rather than one.

In short, there already seems to be an established Falconer tartan, which is readily available (most all tartan shops will have the Keith/Austin/Marshall tartan). Creating a new design for the Falconer tartan would seem to be reinventing the wheel. That, plus it would also seem that there is a titled head of the Falconer family that might have a thing or two to say about it!

Now, if you wanted to design your own personal family tartan for your own use and give it a name that would not imply general use (such as "the Bob Falconer tartan" or "Falconer of Motherwell") then have at it. But it seems to be the best bet would simply be to use the established tartan already available.