An accurate way to write large numbers in Roman numerals is to handle first the thousands, then hundreds, then tens, then units.
Example: the number 1988.
One thousand is M, nine hundred is CM, eighty is LXXX, eight is VIII.
Put it together: MCMLXXXVIII (ⅯⅭⅯⅬⅩⅩⅩⅤⅠⅠⅠ).
Now to just get the no out of ⅯⅭⅯⅬⅩⅩⅩⅤⅠⅠⅠ
To understand any number written in Roman Just use the following tip
M=1000, D=500, C=100, L=50, X=10, V=5 and I=1.
As long as the symbols are in numeric order, you just add the values. However, if a symbol is out of order, you subtract it.
1000 - 100 +1000 + 50 + 10 + 10 + 10 + 5 + 1 + 1 + 1 = 1988.