With 400-405V, 34ma gives you between 13.46 and 13.77w per tube.
36ma @ 400V is 14.4w
36ma @ 405V is 14.58w which is right around 75%.
You can go as high as 36ma comfortably with your voltages. But if 34ma sounds good, why push them, Unless you've got very deep pockets and don't mind purchasing output tubes every year stay between 34ma and 36ma.
Something to consider. Some tubes will drift on bias. So......set them for 34ma and let them cook for a couple weeks checking the ma every couple of days after running it for an hour or so. Write down ALL the voltages on each tube, and the ma's. (Your basically keeping a log of the operation of the tubes). NOTE ANY AND ALL CHANGES. You may have to reset the bias a couple of times before they actually settle down long term. Once they do check once a week for a couple months. If still stable, then once every 6 months. You want to document performance, any changes the tubes have made themselves and what you did to correct it. Seems like a lot of work, but you'll get more life from them this way. Don't go above 75% dissipation or 14.5W per tube.