Calculating the birth Mewa is very simpel if you do know a year with a certain Mewa.
Take for instance the year 1810.
This year has Mewa White 1. As Mewas change in a fixed pattern 1, 2, 3, 4, 5, 6, 7, 8, 9 the next years birth Mewa will be Mewa "Red 9", "White 8", "Red 7", "White 6" "Yellow 5", "Green 4", "Blue 3", "Black 2" and than it repeats itself with the same sequence.
Mewa 1 will move to position 2, 3, 4, etc in the years 1811, 1812, 1813, etc.
If whe need to know the Mewa for a certain year take a reference year. This program uses 1687 which has Mewa Red 7. Take the year of interest minus 1687 add one and divide by 9. The remainder is used to find the correct Mewa. Count from Red 7. E.g. Mewa for 1942 is ( 1942 - 1687 + 1 ) / 9 = 28 rest 4. Start with 1=Red 7, than 2 =White6 3=Yellow5 and 4=Green4 which is the Mewa for 1942.

A complete Mewa Sign Element cycle takes 180 years (metreng) and consists of a Top, Middle and Bottom each a cycle of 60 years (rabjung).
E.g.
1744 - 1803, 1924 - 1983 middle cycle
1804 - 1863, 1984 - 2043 bottom cycle
1864 - 1923, 2044 - 2103 top cycle

The Tibetan Birth Mewa cycle begins in the year of the Mouse as in the Chinese astrology. Every year, besides his birth Mewa also has a Vitality or life force Mewa, a Power or Wealth Mewa, a Body Mewa and a Wind Horse or Luck Mewa.

Once you know the birth Mewa it is easy to find the Vitality, Power and Body Mewa.
The Mewa sequence is a repeating sequence of 123456789.
Assume the birth Mewa is Green 4 ( 1942 Water - Horse ) for the following examples.
The Vitality Mewa is the Mewa which position is 4 places before the birth Mewa. So the Vitality Mewa is White 1. (start with a count of 1 )
The Power Mewa or Wealth Mewa is 4 positions after the birth Mewa. So the Power Mewa is Red 7. (start with a count of 1 )
The Body Mewa is easy as this is the same as the birth Mewa. So Green 4.

The Luck Mewa is more difficult to find.
1. First look after the birthyear sign to find the key for the next step.
2. For a Tiger, Horse or Dog the next step starts with Metal - Monkey
For a Pig, Sheep or Rabbit the next step starts with Fire - Snake
For a Bird, Ox or Snake the next step starts with Water - Pig
For a Mouse, Dragon or Monkey the next step starts with Wood - Tiger
3. Find the key year in the same cycle as the birthyear with the key found in the previous step.
4. The vitality Mewa of this year is the Luck Mewa.

E.g.
1. 1942 Water - Horse
2. Key is Metal - Monkey
3. Metal - Monkey in same cycle is 1980
4. Luck Mewa is Vitality Mewa of 1980 which is White 8