For me, this was one of those things where just experience and practice helped. If you think about it, our bodies have been conditioned from a young age to be like a light switch: either 0% on or 100% on. After I was Penis Enlargementing for a few months, then it was much easier to control my % erect.
BTW, "McGuyver" was a TV show of questionable quality from the 1980s, I think.
Also, be careful which exercises are done at 100% erection. See DLD's excellent exercise descriptions and stick with the recommendations. I've read posts where guys have injured themselves jelquing (I think it was) at 100%. Just search the forums for "bleeding", and I'll bet you find a couple. I'd strongly recommend avoiding all bending exercises while 100%. However, as another poster mentioned, girth exercises mostly need to be done at 100+%.