This is how I dose. I bought a 3.5 gal plastic container with a screw lid to make it seal tight. The container I used was a large chicken waterer from a local feed store. You may be able to find similar sealable containers through lab companies.
I drilled a hole in the bottom of the container, near a corner, to receive a ball valve type fitting and 3/4" inch tubing. Make sure the fitting extends off the bottom of the inside of the jug to keep the undissolved kalk out of your tank and seal the edges with epoxy. This unit will sit above your sump or tank and the 3/4" tubing will be cut so it extends to the water level you wish to maintain in your sump.
I works on the principle of creating a vacuum in the container, so when the sump level drops, air glugs into the container, water comes out until the level is stabilized. The ball valve helps you move the unit around without releasing all the water, it's not used for regulating the flow. I noticed that a large diameter tubing was required or it would air lock, 3/4" to 1" was good.
This has generally worked very well for me, low maintanance and I can leave my tank for a long weekend. My 38 gal loses 1/2 gal a day to evaporation. I use 1 tsp hydrated lime per gal of toppoff water, sometimes more or less depending on how fast my evaporation rate is.
Figuring out your kalk additions will come through trial and error. Start by adding half of what's recommended and test, do this for a few weeks and slowly boost you dosing rate until you get it where you want it.
[ January 05, 2002: Message edited by: fishfarmer ]</p>