Contribution of each variable in multiple linear regression

What will be the best measure of the contribution of a variable in multiple linear regression? I was thinking of using the coefficient ratio as a marker of a variable's contribution.

For example:

If the equation is

${Y}_{predicted}={a}_{1}{X}_{1}+{a}_{2}{X}_{2}+{a}_{3}$

Then ${X}_{1}$'s contribution can be written down as:

$\frac{{a}_{1}}{{a}_{1}+{a}_{2}+{a}_{3}}$

Is there some other method possible to write down the contribution. Since in this case if any coefficient is negative, there is a possibility that a variable's contribution exceeds $100\mathrm{\%}$

What will be the best measure of the contribution of a variable in multiple linear regression? I was thinking of using the coefficient ratio as a marker of a variable's contribution.

For example:

If the equation is

${Y}_{predicted}={a}_{1}{X}_{1}+{a}_{2}{X}_{2}+{a}_{3}$

Then ${X}_{1}$'s contribution can be written down as:

$\frac{{a}_{1}}{{a}_{1}+{a}_{2}+{a}_{3}}$

Is there some other method possible to write down the contribution. Since in this case if any coefficient is negative, there is a possibility that a variable's contribution exceeds $100\mathrm{\%}$