This method was taught to me by an experienced GANN trader
simple rule of PERIOD measurement.
Using previous weeks measurements for the current week.
Using previous months measurements for the current month.

vertical line for start and end of previous week as time. vertical line for start and end of previous month as time.
fibo ret, for high and low of previous week as price (change the default fibo %'s to GANN %'s , 0.0 0.125 0.25 0.375 0.5 0.625 0.75 0.875 1
fibo ret, for high and low of previous month as price.
after getting the correct box of previous week time&price use the trend line to draw angles,grids or both(up to you), but you will know the exact % of the lines.

5 minutes in the weekend to make the measurements, will save you a whole week of guessing. just use simple DOW reading compared with the major levels you have to daytrade or swing.
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I USE FIB FAN with simple 0.0 0.25 0.5 0.75 1 settings.
I was taught to use the monthly as my guide chart and the weekly as my trading chart.
I make my measurement from the start of the week to the start of the next week. I find it easier for the dates.
I will not make my next weekly chart on H1 until the market opens at the start of the next week. Chart dates will be March 25th 00.00 to April 1st 00.00. The new monthly chart on H4 I will also make with the dates March 1st 00.00 to April 1st 00.00