The problem could be two fold, one is that the front sight needs to be lowered which is common.
And the other is your size and how the rifle fits you. If your head and eye doesn't get down far enough on the comb of the stock to be in alignment with the sights, then it could be that your looking down on the sights a little bit.
The front sight should be sighted in with round balls, and then the groups for the heavy conicals could be compared to see how much difference there is.
There is a formula for adjusting the front sight. But the sight would only be adjusted for one load at a particular distance. Any other distance would need to employ some Kentucky windage.
But to reiterate, I wouldn't adjust the front sight soley based on heavy conical loads unless you at least try shooting some groups with round balls too as a comparison.
The formula for front sight adjustment or replacement is:
Record:
error on target in inches
distance to target in inches
sight radius in inches.
divide error on target by distance to target,
multiply by sight radius.
If rear sight is adjustable, adjust to middle of it's adjustment limits to allow a range of adjustment up or down after replacing or modifying front sight.
To use your case as an example, I will assume the sight radius on your pistol is 10 inches
error on target= 12" low.
distance to target is 900".
sight radius is 10".
12" divided by 900" = 0.01333 x 10" = 0.133".
The front sight has to be lowered by 0.133 inches or a bit more than 1/8 of an inch (.125)
I tried to tell you about the desirability of having adjustable sights.
http://thefiringline.com/forums/showpost.php?p=5320174&postcount=19
However all is not lost if you're willing to either install an adjustable sight set on the gun or at least look into your options for either a new front or rear sight.
I don't know the size of your dovetails, but many European guns have a standard 3/8" dovetail or close to it.
And also, the rear sight is moved in the direction of where the impact needs to go, whereas the front sight is moved in the opposite direction of where you want the impact to move.
In this case, you need either a higher rear sight, or you need to lower your front sight.
You need to decide which is better for alignment with your eye when the rifle is shouldered. A new sight or two new sights is not the end of the world. Fixed metal sights can be fairly inexpensive, but sometimes fitting them into the dovetail can require a little work. And some adjustable sights may require that a hole be drilled & tapped for a mounting screw. If you're not handy, go to a gunsmith or a friend who can help you.
But a simple higher rear sight can be filed down to get the gun sighted in, and the notch can be deepened too.
Track of the Wolf has many types of rear sights, some with step elevators for Hawken style rifles, but I don't know if they will work on your rifle or not.
http://www.trackofthewolf.com/Search.aspx?search=rear%20sight
They also have an adjustable metric dovetail sight set for imported rifles, but I don't know if it will fit your narrow barrel diameter either. It's made by Investarms. The 2 rear dovetails have a screw hole in it to mount the rear sight into. The sight be a little wider than your barrel flat, but it may still work or may not.
It's completely removable. Ask for their advice about your model, and which higher rear sight that they would recommend. Even if it a fixed or one with an elevator.
http://www.trackofthewolf.com/Categories/PartDetail.aspx/873/1/RS-LYMAN-FO-SET
Measure the height of your rear sight so that you can compare the height of any other rear sight that you're interested in. I would think that buckhorn or semi-buckhorn rear sights can be relatively taller. They're designed for faster target acquistion for hunting rather than precision target shooting. Look at what they have in that style.
Then call them for advice about different sight heights.
Dixiegun.com is another company that has a lot of sights.