Update EIP-7742: use parent target blob count for excess blob gas cal…

…culation

Merged by EIP-Bot.

EIPS/eip-7742.md

@@ -81,7 +81,17 @@ target blob count given by that genesis block's protocol rule set.
 Upon activating this EIP (i.e. before processing any transactions),
 the verification of the blob maximum as given in EIP-4844 can be skipped. Concretely, this means any logic relating
 to `MAX_BLOB_GAS_PER_BLOCK` as given in EIP-4844 can be deprecated.
-Additionally, any reference to `TARGET_BLOB_GAS_PER_BLOCK` from EIP-4844 can be derived by taking the `target_blobs_per_block` from the CL and multiplying by `GAS_PER_BLOB` as given in EIP-4844.
+
+Additionally, `calc_excess_blob_gas` is updated as follows:
+
+```python
+def calc_excess_blob_gas(parent: Header) -> int:
+    target_blob_gas = parent.target_blobs_per_block * GAS_PER_BLOB
+    if parent.excess_blob_gas + parent.blob_gas_used < target_blob_gas:
+        return 0
+    else:
+        return parent.excess_blob_gas + parent.blob_gas_used - target_blob_gas
+```
 
 Otherwise, the specification of EIP-4844 is not changed. For example, blob base fee accounting and excess blob gas tracking occur in the exact same way.