The technique you will have to use will be a little different than you have described. Basically you are going to have to line up the cams visually with the crank at TDC. I did not have the tool and you can do this without it.
First a little background. Tthe ZDmax/SIR tool is a little different than the OEM tool. The OEM tool locks into the intake cam access hole, not the cam itself, and them fits into the grooves on the exhaust cams and allows you to rotate the cams into position (assumes that they are relatively close to the correct position. So you lock the engine at TDC, fit the tool, loosen the sprocket and rotate the cam to the correct position. then you tighten the sprocket bolts to set the position of the cams. This is done on one side, the the crank is rotated 360 degrees and the other side is done.
With the ZDmax/SIR tool it just locks in the cams with the grooves straight up and down so you have to rotate the crank until the grooves are straight, install the tool, them loosen the bolts on the sprocket, move the crank to TDC and tighten the sprocket bolts. Then do the other side.
First, how far off are you? If you are fairly close my technique will work. You can't really lock the crank at TDC and then loosen the sprocket bolts and turn the cams as with the valve springs, etc. it is hard to rotate the cams, lots of pressure. When I had my cover off I used a pair of vice grips (kind of crude) on a non-critical part of the cam to rotate it close to position, but this is not possible with the covers on. I even tried to make a tool by modifying a socket to fit into the grooves on the end of the exhaust cam, but even with this and a socket wrench it was not possible to keep enough pressure on the socket to keep it in the grooves and rotate the cam.
With the cam cover on and the cams close to the correct position here is the technique that I used. Choose which side you want to do first and get the crank at TDC with the cams on that side close to the correct position. Use a socket on the pulley to rotate the crank ( this is how I did it although Porsche makes a tool that fits into the pulley to rotate the crank). Then rotate the crank slightly to get the groove straight up and down, the crank may not be at TDC. (As an aside, I saw one article that said to never rotate the crank in reverse, but I can't understand why this would damage anything and here we are talking about only a few degrees of rotation.) It can be hard to make sure the grooves are lined up perfectly straight up and down, use a straight edge and the joint between the head and the cam cover as a guide. Once the cam is set loosen the bolts on the sprocket, then rotate the crank to TDC, should only be a few degrees, lock the crank at TDC. Hopefully the cams have not moved. Now tighten the sprocket bolts. I tightened two of them, then put loctite on the other two and tightened them to torque, the loosened the first two and put loctite on them. Now that side should be set. Rotate the crank 360 degrees and use the same technique on the other side. This should get them about as close as you can without the special tool. You need to be fairly precise as 1 degree at the cam is the equivalent of 2 degrees at the crank.
Sorry if this write-up is a little disjointed but it is late and I have cut and pasted it from an email I sent to another owner. Let me know if you have any questions. By the way, if you are not going to remove the cam covers you do not need to buy all the tools in the SIR set, which is very expensive, you only need the tool at the top left of the picture. You can use a torx of the proper diameter to lock the crank. The other tools shown are to hold the cams when removing the cover and when removing the cams.