1/3 full could be too low especially if you do a lot of steaming, 3/4 full is probably too much, and I do not understand what you mean by filling it to the bolt below the handle? I was curious so I checked my Vesubio. Not bolt but it does have fill lines for espresso on the inside. The maximum fill level for 9 cups of espresso is just below the center of the handle. And the the minimum fill level is for 3 cups which is 3/8" above the weld seam at the center of the boiler.
When using the espresso making machines the espresso is made first. This obviously reduces the water level. Then the milk is steamed at this reduced water level. I estimated this as the middle of the boiler. To prove it to myself I filled my machine to the center weld seam and then added 3 cups of water from a standard sized espresso cup. The water reached the 3 cup fill line.
3/4 full may also be too close to the safety valve which is located in the center of the handle. It is important not to fill above the safety valve opening because in the event of an over pressure condition hot water will spray out through the holes in the chrome base of the handle
In addition the steam valve opening is at the same level as the safety valve so you would get water instead of steam
Here is a link to a website that sells the Bellman (a rebadged Vesubio) and has very detailed instructions on how to use it etc:http://www.fantes.com/bellman-parts.html
Interesting, I just re read the website's instructions and under the CAUTIONS it states "never cover the hole of the safety valve when filling the machine with water"
It sure looks like the answer is fill the machine to the middle when steaming.