Everybody is kind of in the ballpark.
The hour representation included on your tach (the odometer) is based strictly on engine speed. The faster the tach turns, the faster the instrument records "time."
The electronic hour meter is dependent on voltage...operate it at a lesser voltage, it runs slower. A higher voltage, it runs higher. it should be reasonably accurate, but not necessarily. I flew a light turboprop last night for 41 minutes and saw .3 register on the electronic hour meter installed in that aircraft. (gotta have a look at it tomorrow).
The electronic hour meter only represents time when it's been activated, and a varietyof means are employed to do this. The switch to activate the hour meter may be on the same circuit as the battery master switch; turn on the electricity, and it starts registering time. It may be wired through an engine oil pressure switch; the meter only runs when the engine is developing a given oil pressure. It may be attached to a landing gear squat switch, or sometimes to a uplock circuit; only when the weight is on the gear, or when the gear is up will the meter register time. Some meters use an air pressure vane or switch to activate the hour meter.
Each of these installations changes the amount of time that the meter will register during any given flight, by changing the time it starts and stops.
The hour meter on your tachometer, the odometer, shows time from engine start, but the rate at which the time is scheduled varies with the engine or propeller speed. At idle, you'll need to sit a lot longer than an hour to register an hour.
The rate at which one may convert "tach hours" to the hours in your logbook will vary with the tach. Some add .2 per hour, others .2 per flight. Some instruments will be 1.4 hours per indicated hour, others 1.2. I've seen wide variations in indications.
Additionally, very few tachs actually read correctly. I sampled a light piston single yesterday and noted that the actual prop RPM was 300 rpm less than indicated. This isn't at all uncommon. The instrument may be measuring at the wrong rate in conjunction with indicating the wrong value.
If you're short a certain value of time based on having referenced the tach vs. the clock, you can certainly correct it. You needn't correct every entry. Simply make one entry for the value you desire...if you need to add 60 hours, make a single entry as though it were a flight, with 60 hours as the value. Then in the remarks column note that it's a time correction for taxi and conversion factor. It's not messy, it's discreet, and nobody is going to fault you.
If you're looking at an airline application, some will disallow for taxi time; some will reduce your total flying, others will add to it. The policies vary. You can't account for them all; simply log your time as professionally as you can, and make any necessary adjustments on an application as the individual employer may require. The employer will let you know.
