What was earth's mass before the collision with theia?
I'm not sure if there was a significant amount of debris, however the calculation you mentioned would at least give an upper bound on the value:

$
M_{piE}=\text{Greatest possible mass of pre-impact Earth}\\
M_E=\text{Mass of Earth today}=6.0\times10^{24}kg\\
M_M=\text{Mass of the Moon}=7.3\times10^{22}kg\\
M_T=\text{Mass of Theia}\approxeq6.4\times10^{23}kg\\
$
$$
\begin{eqnarray*}
M_{piE}&amp;=&amp;M_E+M_M-M_T\\
&amp;=&amp;(6.0\times10^{24})+(7.3\times10^{22})-(6.4\times10^{23})\\
&amp;=&amp;5.4\times10^{24}kg
\end{eqnarray*}
$$
So it wouldn't have been more than about $5.4\times10^{24}kg$.