In the ova 10 of Hellsing Ultimate we get the greates strength feat of Seras in her True Vampire form (therefore her greatest strength feat). It's when she fires a 8.8cm (88mm) Flak 36 anti-aircraft cannon.
http://www.youtube.com/watch?v=BcksEme8wtE - in the video you can see that Major confirms it's 88mm and you can compare cgi model from video to picture on article on wikipedia http://en.wikipedia.org/wiki/88_mm_gun.
The article covers also 18/37/41 models but they are practicly the same. By the way in video is the same one as on the picture on article. Before I start I'll mention that this won't be 100% accurate since it's practicly impossible to be 100% right by guessing parameters from video. Anyway I'll try to be as accurate as possible when speculating about certain parameters.
The weight of the gun is 7407kg. And in video we see that Seras is firing it with one arm(demonic one). Now I must make certain asumptions based on comparison with comic - book level normal human.
We are all familiar with the fact(I hope so) that normal comic book human(coind by Marvel, later also adopted by DC) has strenght level which allows him to military press (overhead press, without leg help) 60kg. This is now changed to "being able to lift one's bodyweight" (not handstand push up, since it's easier to do than lifting 60kg bar with plates above your head) , but for the purpose of this calculation I'll use 60kg . In contrary if i was going to use "bodyweight" than I would have to asume her bodyweight and it would be practicly the same method. By the way she looks like her bodyweight is 60kg. And i asume that this human is skilled in shotgun used, has decent marksman skill, so we exlude "aiming problems" with this.
Above thing was necessary since I'll use human with strenght level of 60kg as a model, and then using proportion I'll calculate Seras's strength.
Since she fired it once and with little difficult i think it's safe to compare it to normal comic book human firing a shotgun with one hand. I'll use 12 gauge Winchester 1897 as a model http://en.wikipedia.org/wiki/Winchester_Model_1897. For all of you who think it's impossible to shoot it with one hand take a look at this video http://www.youtube.com/watch?v=SyBXNRS9czE- I'm not convinces that this guy could military press 60kg overhead. The human who could could fire it more precise than the guy from video(thanks to greater strength). And therefore comicbook normal human can also fire it with more control
To explain it again. I'll asume that my comicbook normal human - let's name him Abraham (like Van Helsing) can shoot Winchester 1897 as skilled as Seras shot Flak36. I'll exclude her "vampiric eye' ability since it's used form marksmanship. I'm scaling gun control here. And to control it better you need some strength. In case of two-hand shooting technique plays a good portion of it, but this is one-hand shooting. With one hand shooting more important is to have strong arm.
Now let's start.
Winchester 1897 weight is 3.6kg, 88mm Flak18/36 weight is 7407kg. Abraham's stenght level is 60kg, Seras's strength level is unknown or X. Abraham with strength level of 60kg can fire Winchester 1897 one-handed with same control as Seras did 88mm. Proportion would be ABRAHAM'S STENGHT LEVEL : WEIGHT OF SHOTGUN = SERAS'S STRENGTH LEVEL : WEIGHT OF 88MM . Therefore Sera's strenght level is = (Abraham's strength level x weight of 88mm)/weight of shotgun <=> X=(60 x 7407)/3.6 <=> X=123450kg or X=123.450 tons.
Therefore Seras's strength level is slightly above 100tons or 123.45 tons. With those asumptions I got expected result, . This site did most calculations about Hellsing and they put Seras at similar level (100tons) http://hellsingultimate.wikispaces.com/Seras+Victoria. I said expected because I was calculating it while typing and didn't made parameters to fit to result of the site. I USED SHOTGUN AND HUMAN STRENGTH LEVEL RANDOMLY(i typed this in caps look to be seen better).
P.S. Sorry if text was long, but I tried to explain everything and cover every possibility. Given the fact that I was guessing parameters for calc (there was no way to measure them or scale them from video, since we talk about strength, control while shotting, overpowering recoil...) i think calc is good, anyway I want to hear other opinions as well . So what you think about it ?