Q:

How to measure angle when ratcheting a bolt?

The torque spec for my Tacoma says that cylinder head bolts should be torqued (in a specific sequence) to 27 ft/lbs initially and then again each should be turned another 180 degrees in the same sequence.
I know how to use a torque wrench to set torque to a specific ft/lbs but I don't know how to measure the angle turned by ratcheting.  How do I make sure that a certain number of clicks on a ratchet equates a certain angle?
As a bonus question, can you explain why Toyota would spec the second round of torquing in angles and not ft/lbs ?  I think ft/lbs would be a cleaner spec.

A:

90° = quarter turn. 180° = half turn. It's alright if you are off a few degrees.  
I typically start with the breaker bar perpendicular (straight out) and do quarter turns, or have it straight off to the left. Just keep yourself parallel or perpendicular to where you start.
DO NOT USE A TORQUE WRENCH. It's bad for the torque wrench to turn after its clicked.  Use a breaker bar.
These bolts are "torque to yield." They stretch out, which is why they should not be reused.
If you want to be precise you can use a Torque Angle Gauge.

A:

I'm going to buck the other two answers and tell you DO IT RIGHT OR DON'T DO IT. 
It is very important to torque your head bolts correctly. The actual preferred method for attaining the proper torque is by figuring out the fastener stretch, not by using a torque wrench. When torquing, you are applying a clamping load on the object you are torquing. You can get this at its most accurate by measuring the stretch of the fastener by using a dial indicator. It's by far easier to torque the fastener down using a torque wrench and in most applications, this is an adequate method of doing things.
In your case it seems you are using torque-to-yield (T2Y) fasteners. These are (as @rpmerf pointed out) single use fasteners only (NOTE: If I could make this text blink, I would to emphasize the point). These must be replaced once you have broken them free (this would blink as well). If you don't, your head gasket will fail. The head gasket is the typical place for these types of fasteners. The reason these fasteners are only good for one go around is they are designed to go up to that "fail point", but not beyond. They start to fail by giving a bit or stretching. These are specific built fasteners by the manufacturer. They have done the stretch engineering for you. By applying the last degree turn, you are stretching the bolt to the tolerance they have engineered into the bolt. If you go past this point, you are only courting death for your engine. If you don't go far enough, you'll have a weak point in your torque chain which could allow for warpage in the part you are trying to affix ... especially true for a head.
As rpmerf stated, get the torque angle gauge:

A:

Torque the bolt to the required torque.  Then mark the head of the bolt with a marker or pen.  This way you can visually check the angle.  This is commonly used to ensure bolts are torqued during assembly.

