This Minecraft Forge mod provides a HUD next to the reticle showing the distance to the block that is being pointed at.
Download the desired version of Forge MDK from https://files.minecraftforge.net/ and unzip the MDK into a new dirctory. After the MDK is unzipped, remove the main folder from the src folder and clone this repo into the src directory as main. Then you will need to either copy or link the build.gradle from the repository to the root of the MDK, replacing the original one.
Replace <MC_VERSION> with the Minecraft version of the MDK (for example ~/mdk_1.10.2) and <MDK_FILE> with the file name of the MDK you downloaded (for example forge-1.10.2-12.18.2.2099-mdk.zip)
mkdir ~/mdk_<MC_VERSION>
cd ~/mdk_<MC_VERSION>
cp <MDK_FILE> .
unzip <MDK_FILE>
rm -rf src/main
git clone https://github.com/ToroCraft/DistanceMeter.git
mv build.gradle build.default.gradle
ln -s src/main/build.gradle build.gradle
./gradlew setupDecompWorkspace
./gradlew eclipse