What I do is look at the height of the enlarging head on the chasis. Many chasi's have a scale on one of the posts, or the post, somewhere.. you dont need to be acurate as far as what height the film actually is, just consistent (precise) on how you read the height. On the Beseler MX45, I always read the height of the motor switch (but any part of the machine that goes up or down with the film will do).
If say, I do an enlargment and the (enter your choice of "marker" here) switch is at 8inches, and I then decide to make things larger, say I raise the head so that my marker is now reading 11" on the post, I double the exposure. It's not dead acurate, but it gets me whithin one print of where I want to be.
What it relies on is that the way light falls off is consistent whether it's going into the camera, or comming out of it. As the distance increases, the light falls off at a predictable rate. I just choose to use aproximate f/stop height readings to judge more or less how much to change exposure times. If I started with the enlarger head at 44", and dropped it down to 6", I'd cut exposure by the same # of stops between f/5.6 and f/45 . . . It's harder to write about than to actually do... Just think about the way light is behaving, and what it's doing as you raise and lower the head. If the times get out of hand b/c of raising or lowering the head, I finish the adjustments with the lens aperture control. It's something I've been trying out to cut down on wated paper myself.. and it seems to be giving me workable approximations. YMMV.