There is no conversion. The reason is, the degree requirement implies a need to stretch the bolt. You cannot do that with torque alone. The initial torque is to ensure you are at the proper starting point, then the degrees gets the stretch done.
If you do not have an angle guage, the manufacturer was nice enough to actually give you a range of degrees to work with. The "+/- 5°" gives you a fairly broad range. The easiest way to do this without the angle guage is actually by eye. To accomplish this, first, ensure the engine crankshaft is locked in place and will not turn over. Next, torque the bolt to the 30nm to get it set at the initial torque. Then, with a bright marker (something clearly visible on the dark metal), mark a hash line on the bolt head so it is pointing directly up at 12 O'Clock. You only want this mark on the edge of the head of the bolt. Once marked, this should give you a clear indication of how far the bolt is turned. Since 90° is a right angle from where you started, this is very easily discernable. You'll need to go just past this, enough to show the naked eye you are past it, but not by much. This should get you to at least 95° which is within the spec. If you feel froggy and go past that point a bit, still no big deal, because you've got 10° of play to work with. You can also measure the 90° by way of the device you're using to turn the bolt. If you start it at straight up, you can see how far you've turned the bolt, then take it off the bolt and see where the line is at. Your mark should end up between the 3 and 4 O'clock position. Please note, you want to err on the side of caution and not turn the bolt too far. Reason is, there's no turning back, or rather, you can't back the bolt off to get back to the proper degree of turning the bolt.