What is the empirical formula of Haemoglobin?
Unfortunately this is only half an answer. I'd be thankful if someone could chip in with the rest.

When you want structural information about proteins, the first place to look is the Protein Data Bank, a freely accessible repository of protein structures determined by techniques such as X-ray crystallography, protein NMR and neutron diffraction, among others.

The problem now is being sure about what you want to find. A basic search for human hemoglobin currently provides 190 hits, so you need to browse a bit to find the most adequate entry. To keep things simple, I looked for a structure for adult hemoglobin, with no mutations, ligands or associations with other proteins. There are some database entries which seem to be right on target, namely 2DN2 for deoxyhemoglobin and 2DN1 for oxyhemoglobin.

There is a lot of information in each entry page. The whole structure can be downloaded as a .pdb and opened in appropriate structure visualization programs, of which there are probably several free options. Depending on the program used, retrieving the empirical formula can be trivial.

Forgoing the use of molecular visualization software, there is an "Experimental Data &amp; Validation" section in the protein page, containing a "Full Validation Report" link to a PDF file. There, you can find a section called "Entry Composition" with useful information. For example, in the deoxyhemoglobin structure there are 4777 non-hydrogen atoms. The protein is composed of two copies each of two unique subunits (the alpha and beta subunits), totalling four subunits. The alpha subunit contains 1069 non-hydrogen atoms (685 $\ce{C}$, 187 $\ce{N}$, 194 $\ce{O}$ and 3 $\ce{S}$), while the beta subunit contains 1123 non-hydrogen atoms (724 $\ce{C}$, 195 $\ce{N}$, 201 $\ce{O}$ and 3 $\ce{S}$). Additionally, each subunit also contains one molecule of protoporphyrin IX coordinated to an iron atom. One important thing to note is that the structure also contains solvent molecules, in this case a total of 221 molecules of water, which must be removed from the atom count.

The only flaw is that hydrogen atoms are not shown. This is because X-ray crystallography measurements generally cannot detect hydrogen atoms, and so they are usually added separately to the backbone of the protein, in such a way to complete all atom valencies. I don't know how this can be done other than using a program.

A last thing to keep in mind is that no matter how good the measurement or the data processing, you can very well end up a few atoms off, especially regarding hydrogen atoms. Hopefully perfect precision is not required.